diff --git a/sparkth/api/v1/auth.py b/sparkth/api/v1/auth.py
index 29347fba..279c9249 100644
--- a/sparkth/api/v1/auth.py
+++ b/sparkth/api/v1/auth.py
@@ -25,6 +25,7 @@
 from sparkth.lib.audit.context import AnonymousActor, UserActor
 from sparkth.lib.audit.events import AuditOutcome, AuditTarget, LoginAuditEvent
 from sparkth.lib.db import get_async_session
+from sparkth.lib.i18n import _
 from sparkth.schemas import (
     GoogleAuthUrl,
     ResendVerificationRequest,
@@ -54,23 +55,23 @@ async def register_user(
     session: AsyncSession = Depends(get_async_session),
 ) -> User:
     if not settings.REGISTRATION_ENABLED:
-        raise HTTPException(status_code=403, detail="Registration is currently disabled")
+        raise HTTPException(status_code=403, detail=_("Registration is currently disabled"))
 
     normalized_email = user.email.strip().lower()
     if not await WhitelistService.is_email_allowed(session, normalized_email):
         raise HTTPException(
             status_code=status.HTTP_403_FORBIDDEN,
-            detail="This email address is not authorized to register. Contact an administrator.",
+            detail=_("This email address is not authorized to register. Contact an administrator."),
         )
     result = await session.exec(select(User).where(User.username == user.username))
     db_user = result.one_or_none()
     if db_user:
-        raise HTTPException(status_code=400, detail="Username already registered")
+        raise HTTPException(status_code=400, detail=_("Username already registered"))
 
     result = await session.exec(select(User).where(User.email == normalized_email))
     db_user_email = result.one_or_none()
     if db_user_email:
-        raise HTTPException(status_code=400, detail="Email already registered")
+        raise HTTPException(status_code=400, detail=_("Email already registered"))
 
     hashed_password = security.get_password_hash(user.password)
     db_user = User(
@@ -140,7 +141,7 @@ async def login_for_access_token(
         )
         raise HTTPException(
             status_code=401,
-            detail="Incorrect username or password",
+            detail=_("Incorrect username or password"),
             headers={"WWW-Authenticate": "Bearer"},
         )
 
@@ -155,7 +156,7 @@ async def login_for_access_token(
         )
         raise HTTPException(
             status_code=401,
-            detail="Incorrect username or password",
+            detail=_("Incorrect username or password"),
             headers={"WWW-Authenticate": "Bearer"},
         )
 
diff --git a/sparkth/api/v1/file_parser.py b/sparkth/api/v1/file_parser.py
index da600de5..ac063594 100644
--- a/sparkth/api/v1/file_parser.py
+++ b/sparkth/api/v1/file_parser.py
@@ -6,6 +6,7 @@
 
 from sparkth.core.models.user import User
 from sparkth.lib.auth import get_current_user
+from sparkth.lib.i18n import _
 
 router: APIRouter = APIRouter()
 
@@ -38,12 +39,12 @@ async def parse_pdf(content: bytes) -> str:
 @router.post("/upload")
 async def upload_text(current_user: User = Depends(get_current_user), file: UploadFile = File(...)) -> JSONResponse:
     if not current_user or not current_user.id:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not authenticated.")
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail=_("User not authenticated."))
 
     if file.content_type not in ALLOWED_TYPES:
         raise HTTPException(
             status_code=400,
-            detail="Only .txt and .pdf files are supported",
+            detail=_("Only .txt and .pdf files are supported"),
         )
 
     content = await file.read()
@@ -52,7 +53,7 @@ async def upload_text(current_user: User = Depends(get_current_user), file: Uplo
     if size > MAX_FILE_SIZE:
         raise HTTPException(
             status_code=400,
-            detail="File size exceeds 30MB limit",
+            detail=_("File size exceeds 30MB limit"),
         )
 
     if file.content_type == "text/plain":
@@ -62,7 +63,7 @@ async def upload_text(current_user: User = Depends(get_current_user), file: Uplo
         text = await parse_pdf(content)
 
     else:
-        raise HTTPException(status_code=400, detail="Unsupported file type")
+        raise HTTPException(status_code=400, detail=_("Unsupported file type"))
 
     return JSONResponse(
         {
diff --git a/sparkth/api/v1/llm.py b/sparkth/api/v1/llm.py
index f29faf80..69a84d02 100644
--- a/sparkth/api/v1/llm.py
+++ b/sparkth/api/v1/llm.py
@@ -7,6 +7,7 @@
 from sparkth.core.models.user import User
 from sparkth.lib.auth import get_current_user
 from sparkth.lib.db import get_async_session
+from sparkth.lib.i18n import _
 from sparkth.lib.llm import (
     DEFAULT_MODEL,
     DEFAULT_PROVIDER,
@@ -93,7 +94,9 @@ async def update_llm_config(
     service: LLMConfigService = Depends(get_llm_service),
 ) -> LLMConfigResponse:
     if not body.name and not body.model:
-        raise HTTPException(status_code=status.HTTP_422_UNPROCESSABLE_CONTENT, detail="Empty body. Nothing to update.")
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT, detail=_("Empty body. Nothing to update.")
+        )
 
     try:
         config = await service.update(
@@ -173,5 +176,5 @@ async def delete_llm_config(
         config_id=config_id,
     )
     if not deleted:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="LLM config not found")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=_("LLM config not found"))
     await session.commit()
diff --git a/sparkth/api/v1/user/routes.py b/sparkth/api/v1/user/routes.py
index 541abc22..d51d8c52 100644
--- a/sparkth/api/v1/user/routes.py
+++ b/sparkth/api/v1/user/routes.py
@@ -20,6 +20,7 @@
 from sparkth.core.models.user import User
 from sparkth.lib.auth import get_current_user
 from sparkth.lib.db import get_async_session
+from sparkth.lib.i18n import _
 from sparkth.lib.permissions import has_role
 from sparkth.lib.permissions.scopes import GLOBAL
 from sparkth.schemas import User as UserSchema
@@ -76,7 +77,7 @@ async def update_user_language(
         # that dependency: an authenticated principal without a row is an
         # authentication-boundary condition, not a domain error to route through the
         # exception-handler registry.
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="User not found")
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=_("User not found"))
 
     user.language = update.language
     # updated_at has a default_factory but no onupdate, so the bump is manual.
diff --git a/sparkth/api/v1/user_plugins.py b/sparkth/api/v1/user_plugins.py
index 31bb513c..2534694a 100644
--- a/sparkth/api/v1/user_plugins.py
+++ b/sparkth/api/v1/user_plugins.py
@@ -22,6 +22,7 @@
 )
 from sparkth.lib.auth import get_current_user
 from sparkth.lib.db import get_async_session
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 
 logger = get_logger(__name__)
@@ -45,7 +46,9 @@ class UserPluginConfigRequest(BaseModel):
 async def get_plugin_or_404(plugin_service: PluginService, session: AsyncSession, name: str) -> Plugin:
     plugin = await plugin_service.get_by_name(session, name)
     if not plugin:
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Plugin '{name}' not found")
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail=_("Plugin '{name}' not found").format(name=name)
+        )
     return plugin
 
 
@@ -55,7 +58,9 @@ def validate_plugin(plugin: Plugin) -> None:
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail=f"Plugin '{plugin.name}' is not persisted."
         )
     if not plugin.enabled:
-        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=f"Plugin '{plugin.name}' is not enabled")
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN, detail=_("Plugin '{name}' is not enabled").format(name=plugin.name)
+        )
 
 
 @router.get("/", response_model=list[UserPluginResponse])
@@ -115,7 +120,7 @@ async def create_user_plugin(
 ) -> UserPluginResponse:
     """Create a user plugin with validated configuration."""
     if not current_user.id:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not authenticated.")
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail=_("User not authenticated."))
 
     plugin = await get_plugin_or_404(plugin_service, session, plugin_name)
     validate_plugin(plugin)
@@ -123,7 +128,8 @@ async def create_user_plugin(
     user_plugin = await plugin_service.get_user_plugin(session, current_user.id, plugin.id)
     if user_plugin and len(user_plugin.config) > 0:
         raise HTTPException(
-            status_code=status.HTTP_409_CONFLICT, detail=f"Plugin '{plugin_name}' is already configured"
+            status_code=status.HTTP_409_CONFLICT,
+            detail=_("Plugin '{name}' is already configured").format(name=plugin_name),
         )
 
     try:
@@ -189,7 +195,7 @@ async def update_user_plugin(
     Enable or disable a plugin for the current user.
     """
     if not current_user.id:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not authenticated.")
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail=_("User not authenticated."))
 
     plugin = await get_plugin_or_404(plugin_service, session, plugin_name)
     validate_plugin(plugin)
@@ -219,7 +225,7 @@ async def update_user_plugin_config(
     This allows users to customize plugin-specific settings.
     """
     if not current_user.id:
-        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not authenticated.")
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail=_("User not authenticated."))
 
     plugin = await get_plugin_or_404(plugin_service, session, plugin_name)
     validate_plugin(plugin)
diff --git a/sparkth/core/google_auth.py b/sparkth/core/google_auth.py
index 56231e45..771424d9 100644
--- a/sparkth/core/google_auth.py
+++ b/sparkth/core/google_auth.py
@@ -6,6 +6,7 @@
 import aiohttp
 
 from sparkth.core.config import get_settings
+from sparkth.lib.i18n import _
 
 # Google OAuth endpoints
 GOOGLE_AUTH_URL = "https://accounts.google.com/o/oauth2/v2/auth"
@@ -37,7 +38,7 @@ def generate_google_login_url() -> str:
     Returns:
         The authorization URL to redirect the user to.
     """
-    client_id, _, redirect_uri = get_google_credentials()
+    client_id, _client_secret, redirect_uri = get_google_credentials()
 
     params = {
         "client_id": client_id,
@@ -100,6 +101,6 @@ async def get_google_user_info(access_token: str) -> dict[str, Any]:
         headers = {"Authorization": f"Bearer {access_token}"}
         async with session.get(GOOGLE_USERINFO_URL, headers=headers) as response:
             if response.status != 200:
-                raise ValueError("Failed to get user info from Google")
+                raise ValueError(_("Failed to get user info from Google"))
             result: dict[str, Any] = await response.json()
             return result
diff --git a/sparkth/core/permissions/__init__.py b/sparkth/core/permissions/__init__.py
index 4b013930..5da5fdb8 100644
--- a/sparkth/core/permissions/__init__.py
+++ b/sparkth/core/permissions/__init__.py
@@ -39,6 +39,7 @@
 from sparkth.lib.auth import get_current_user
 from sparkth.lib.db import get_async_session
 from sparkth.lib.hooks import SingleNamedItemHook
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 
 logger = get_logger(__name__)
@@ -89,7 +90,7 @@ async def dependency(
                 raise RuntimeError("Permission scope is misconfigured")
             scope_object_id = request.path_params.get(scope_param) if scope_param else None
             if not await can(current_user, self, permission_scope, scope_object_id, session):
-                raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Permission denied")
+                raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=_("Permission denied"))
             return current_user
 
         return dependency
diff --git a/sparkth/core/permissions/exceptions.py b/sparkth/core/permissions/exceptions.py
index 4d1239e7..54791e26 100644
--- a/sparkth/core/permissions/exceptions.py
+++ b/sparkth/core/permissions/exceptions.py
@@ -1,8 +1,11 @@
+from sparkth.lib.i18n import _
+
+
 class RoleNotFound(Exception):
     """Raised when a role referenced by name does not exist."""
 
     def __init__(self, role_name: str) -> None:
-        super().__init__(f"Role not found: {role_name}")
+        super().__init__(_("Role not found: {role_name}").format(role_name=role_name))
         self.role_name = role_name
 
 
@@ -10,7 +13,7 @@ class PermissionNotFound(Exception):
     """Raised when a permission referenced by name is not registered."""
 
     def __init__(self, permission: str) -> None:
-        super().__init__(f"Permission not found: {permission}")
+        super().__init__(_("Permission not found: {permission}").format(permission=permission))
         self.permission = permission
 
 
@@ -18,7 +21,7 @@ class PermissionScopeNotFound(Exception):
     """Raised when a permission scope referenced by name is not registered."""
 
     def __init__(self, name: str) -> None:
-        super().__init__(f"Permission scope not found: {name}")
+        super().__init__(_("Permission scope not found: {name}").format(name=name))
         self.name = name
 
 
@@ -26,7 +29,7 @@ class RoleAlreadyExists(Exception):
     """Raised when creating or renaming a role to a name that is already taken."""
 
     def __init__(self, name: str) -> None:
-        super().__init__(f"Role already exists: {name}")
+        super().__init__(_("Role already exists: {name}").format(name=name))
         self.name = name
 
 
@@ -34,7 +37,7 @@ class RoleInUse(Exception):
     """Raised when deleting a role that still has active assignments."""
 
     def __init__(self, role_id: int) -> None:
-        super().__init__(f"Role is still assigned and cannot be deleted: {role_id}")
+        super().__init__(_("Role is still assigned and cannot be deleted: {role_id}").format(role_id=role_id))
         self.role_id = role_id
 
 
@@ -42,7 +45,7 @@ class GroupNotFound(Exception):
     """Raised when a group referenced by id or name does not exist."""
 
     def __init__(self, name: str) -> None:
-        super().__init__(f"Group not found: {name}")
+        super().__init__(_("Group not found: {name}").format(name=name))
         self.name = name
 
 
@@ -50,7 +53,7 @@ class GroupAlreadyExists(Exception):
     """Raised when creating or renaming a group to a name that is already taken."""
 
     def __init__(self, name: str) -> None:
-        super().__init__(f"Group already exists: {name}")
+        super().__init__(_("Group already exists: {name}").format(name=name))
         self.name = name
 
 
@@ -58,7 +61,9 @@ class GroupInUse(Exception):
     """Raised when deleting a group that still has active role assignments."""
 
     def __init__(self, group_id: int) -> None:
-        super().__init__(f"Group still has active role assignments and cannot be deleted: {group_id}")
+        super().__init__(
+            _("Group still has active role assignments and cannot be deleted: {group_id}").format(group_id=group_id)
+        )
         self.group_id = group_id
 
 
@@ -72,6 +77,10 @@ class InvalidScopeObjectId(Exception):
     """
 
     def __init__(self, scope: str, scope_object_id: str | None) -> None:
-        super().__init__(f"Invalid scope/object-id pairing for scope {scope!r}: {scope_object_id!r}")
+        super().__init__(
+            _("Invalid scope/object-id pairing for scope {scope!r}: {scope_object_id!r}").format(
+                scope=scope, scope_object_id=scope_object_id
+            )
+        )
         self.scope = scope
         self.scope_object_id = scope_object_id
diff --git a/sparkth/core/plugins/middleware.py b/sparkth/core/plugins/middleware.py
index 41d8e462..855d89bb 100644
--- a/sparkth/core/plugins/middleware.py
+++ b/sparkth/core/plugins/middleware.py
@@ -15,6 +15,7 @@
 from sparkth.core.routes import get_route_plugin_name
 from sparkth.lib.auth import decode_token_username, get_user_by_username
 from sparkth.lib.db import session_scope
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 
 logger = get_logger(__name__)
@@ -96,8 +97,10 @@ async def dispatch(self, request: Request, call_next: Callable[[Request], Awaita
             return JSONResponse(
                 status_code=status.HTTP_403_FORBIDDEN,
                 content={
-                    "detail": f"Access to plugin '{plugin_name}' is disabled for your account. "
-                    f"Please enable the plugin in your settings."
+                    "detail": _(
+                        "Access to plugin '{name}' is disabled for your account. "
+                        "Please enable the plugin in your settings."
+                    ).format(name=plugin_name)
                 },
             )
 
diff --git a/sparkth/core/plugins/service.py b/sparkth/core/plugins/service.py
index 9d41f8a2..a1ec6b81 100644
--- a/sparkth/core/plugins/service.py
+++ b/sparkth/core/plugins/service.py
@@ -12,6 +12,8 @@
 from sparkth.lib.db import session_scope
 from sparkth.lib.frontend import get_plugin_display_info, get_plugin_sidebar_entry, plugin_has_frontend
 from sparkth.lib.frontend.hooks import DisplayInfo, SidebarEntry
+from sparkth.lib.i18n import _
+from sparkth.lib.i18n import gettext as translate
 from sparkth.lib.log import get_logger
 
 logger = get_logger(__name__)
@@ -48,14 +50,25 @@ class UserPluginResponse(pydantic.BaseModel):
 
     @classmethod
     def for_plugin(cls, plugin_name: str, enabled: bool, config: dict[str, Any], is_core: bool) -> "UserPluginResponse":
-        """Build a response carrying the frontend metadata declared for ``plugin_name``."""
+        """Build a response carrying the frontend metadata declared for ``plugin_name``.
+
+        The declared display and sidebar strings are ``gettext_noop``-marked
+        source messages; this is their rendering boundary, so they are
+        translated into the request locale here.
+        """
+        display = get_plugin_display_info(plugin_name)
+        if display is not None:
+            display = DisplayInfo(translate(display.display_name), translate(display.description), display.icon)
+        sidebar = get_plugin_sidebar_entry(plugin_name)
+        if sidebar is not None:
+            sidebar = SidebarEntry(translate(sidebar.label), sidebar.icon, sidebar.order)
         return cls(
             plugin_name=plugin_name,
             enabled=enabled,
             config=config,
             is_core=is_core,
-            display=get_plugin_display_info(plugin_name),
-            sidebar=get_plugin_sidebar_entry(plugin_name),
+            display=display,
+            sidebar=sidebar,
             has_frontend=plugin_has_frontend(plugin_name),
         )
 
@@ -92,13 +105,13 @@ def validate_user_config(plugin: Plugin, user_config: dict[str, Any]) -> dict[st
         config_class = get_plugin_config_schema(plugin.name)
         if not config_class:
             logger.error(f"Plugin '{plugin.name}' config class is missing or invalid")
-            raise InternalServerError(f"Plugin '{plugin.name}' cannot be configured at this time.")
+            raise InternalServerError(_("Plugin '{name}' cannot be configured at this time.").format(name=plugin.name))
 
         if not issubclass(config_class, PluginConfig):
             logger.error(
                 f"'{plugin.name.title()}Config' must inherit from sparkth.core.plugins.config_base.PluginConfig"
             )
-            raise InternalServerError(f"Plugin '{plugin.name}' cannot be configured at this time.")
+            raise InternalServerError(_("Plugin '{name}' cannot be configured at this time.").format(name=plugin.name))
 
         try:
             validated_config = config_class(**user_config)
@@ -301,7 +314,7 @@ async def update_user_plugin_config(
 
         if user_plugin:
             if not user_plugin.enabled:
-                raise PluginDisabledError("Cannot update plugin configuration while the plugin is disabled")
+                raise PluginDisabledError(_("Cannot update plugin configuration while the plugin is disabled"))
             merged_config = {**user_plugin.config, **user_config}
         else:
             merged_config = user_config
diff --git a/sparkth/lib/auth.py b/sparkth/lib/auth.py
index f4691d15..07793103 100644
--- a/sparkth/lib/auth.py
+++ b/sparkth/lib/auth.py
@@ -22,6 +22,7 @@
 from sparkth.core import security
 from sparkth.core.models.user import User
 from sparkth.lib.db import get_async_session
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 
 logger = get_logger(__name__)
@@ -86,7 +87,7 @@ async def get_current_user(
     if username is None:
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Could not validate credentials",
+            detail=_("Could not validate credentials"),
             headers={"WWW-Authenticate": "Bearer"},
         )
 
@@ -94,7 +95,7 @@ async def get_current_user(
     if user is None:
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="User not found",
+            detail=_("User not found"),
             headers={"WWW-Authenticate": "Bearer"},
         )
 
diff --git a/sparkth/llm/adapter.py b/sparkth/llm/adapter.py
index c3b11b3c..408830f4 100644
--- a/sparkth/llm/adapter.py
+++ b/sparkth/llm/adapter.py
@@ -6,6 +6,7 @@
 from sqlmodel.ext.asyncio.session import AsyncSession
 
 from sparkth.core.models.llm import LLMConfig
+from sparkth.lib.i18n import _
 from sparkth.llm.exceptions import LLMConfigValidationError
 from sparkth.llm.providers import get_models_for_provider
 
@@ -66,14 +67,15 @@ async def preprocess_config(
         if llm is None:
             raise ValueError(f"llm_config_id {config_id} not found or does not belong to this user.")
         if not llm.is_active:
-            raise ValueError("This record is deactivated. Reactivate it in AI Keys or select a different config.")
+            raise ValueError(_("This record is deactivated. Reactivate it in AI Keys or select a different config."))
 
         if model_override is not None:
             allowed = get_models_for_provider(llm.provider)
             if model_override not in allowed:
                 raise LLMConfigValidationError(
-                    f"Model '{model_override}' not available for provider '{llm.provider}'. "
-                    f"Allowed: {', '.join(allowed)}"
+                    _("Model '{model}' not available for provider '{provider}'. Allowed: {allowed}").format(
+                        model=model_override, provider=llm.provider, allowed=", ".join(allowed)
+                    )
                 )
 
         return {**incoming_config, "llm_config_id": config_id}
diff --git a/sparkth/llm/exceptions.py b/sparkth/llm/exceptions.py
index 6822dd08..71618b5d 100644
--- a/sparkth/llm/exceptions.py
+++ b/sparkth/llm/exceptions.py
@@ -1,8 +1,13 @@
+from sparkth.lib.i18n import _
+
+
 class LLMConfigNotFoundError(ValueError):
     """Raised when an LLM config is not found."""
 
     def __init__(self, config_id: int, user_id: int) -> None:
-        super().__init__(f"LLMConfig {config_id} not found for user {user_id}")
+        super().__init__(
+            _("LLMConfig {config_id} not found for user {user_id}").format(config_id=config_id, user_id=user_id)
+        )
 
 
 class LLMConfigModelNotSetError(ValueError):
@@ -18,8 +23,10 @@ class LLMConfigInactiveError(ValueError):
 
     def __init__(self) -> None:
         super().__init__(
-            "The selected AI configuration is deactivated. "
-            "Go to AI Keys to reactivate it, or choose a different configuration in the chat settings."
+            _(
+                "The selected AI configuration is deactivated. "
+                "Go to AI Keys to reactivate it, or choose a different configuration in the chat settings."
+            )
         )
 
 
@@ -27,4 +34,4 @@ class LLMConfigDuplicateNameError(ValueError):
     """Raised when an LLM config with the same name already exists for the user."""
 
     def __init__(self, name: str) -> None:
-        super().__init__(f"An LLM config with name '{name}' already exists for this user.")
+        super().__init__(_("An LLM config with name '{name}' already exists for this user.").format(name=name))
diff --git a/sparkth/llm/service.py b/sparkth/llm/service.py
index 26f1129c..26872d08 100644
--- a/sparkth/llm/service.py
+++ b/sparkth/llm/service.py
@@ -7,6 +7,7 @@
 from sqlmodel.ext.asyncio.session import AsyncSession
 
 from sparkth.core.models.llm import LLMConfig
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 from sparkth.llm.exceptions import (
     LLMConfigDuplicateNameError,
@@ -123,7 +124,9 @@ async def update(
             allowed = get_models_for_provider(config.provider)
             if model not in allowed:
                 raise LLMConfigValidationError(
-                    f"Model '{model}' not available for provider '{config.provider}'. Allowed: {', '.join(allowed)}"
+                    _("Model '{model}' not available for provider '{provider}'. Allowed: {allowed}").format(
+                        model=model, provider=config.provider, allowed=", ".join(allowed)
+                    )
                 )
             config.model = model
         config.update_timestamp()
diff --git a/sparkth/services/email_verification.py b/sparkth/services/email_verification.py
index f167ba2e..53b4ec83 100644
--- a/sparkth/services/email_verification.py
+++ b/sparkth/services/email_verification.py
@@ -11,6 +11,7 @@
 from sparkth.core.email import send_email
 from sparkth.core.models.email_verification import EmailVerificationToken
 from sparkth.core.models.user import User
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 
 settings = get_settings()
@@ -118,19 +119,18 @@ def _build_verify_url(raw_token: str) -> str:
 def _render_email(name: str, raw_token: str) -> tuple[str, str]:
     url = _build_verify_url(raw_token)
     ttl = settings.EMAIL_VERIFICATION_TOKEN_TTL_HOURS
-    text = (
-        f"Hi {name},\n\n"
-        "Click the link below to confirm your email address:\n\n"
-        f"{url}\n\n"
-        f"This link expires in {ttl} hours.\n\n"
-        "If you didn't sign up for Sparkth, you can ignore this email.\n"
-    )
+    greeting = _("Hi {name},").format(name=name)
+    instruction = _("Click the link below to confirm your email address:")
+    expiry = _("This link expires in {ttl} hours.").format(ttl=ttl)
+    footer = _("If you didn't sign up for Sparkth, you can ignore this email.")
+    link_label = _("Confirm my email")
+    text = f"{greeting}\n\n{instruction}\n\n{url}\n\n{expiry}\n\n{footer}\n"
     html = (
-        f"<p>Hi {html_escape(name)},</p>"
-        "<p>Click the link below to confirm your email address:</p>"
-        f'<p><a href="{html_escape(url, quote=True)}">Confirm my email</a></p>'
-        f"<p>This link expires in {ttl} hours.</p>"
-        "<p>If you didn't sign up for Sparkth, you can ignore this email.</p>"
+        f"<p>{html_escape(greeting)}</p>"
+        f"<p>{html_escape(instruction)}</p>"
+        f'<p><a href="{html_escape(url, quote=True)}">{html_escape(link_label)}</a></p>'
+        f"<p>{html_escape(expiry)}</p>"
+        f"<p>{html_escape(footer)}</p>"
     )
     return text, html
 
@@ -143,7 +143,7 @@ async def send_verification_email(*, to: str, name: str, raw_token: str) -> None
     try:
         await send_email(
             to=to,
-            subject="Confirm your Sparkth account",
+            subject=_("Confirm your Sparkth account"),
             html_body=html_body,
             text_body=text_body,
         )
diff --git a/sparkth/services/whitelist/service.py b/sparkth/services/whitelist/service.py
index 270c4c72..b8b02966 100644
--- a/sparkth/services/whitelist/service.py
+++ b/sparkth/services/whitelist/service.py
@@ -5,6 +5,7 @@
 from sqlmodel.ext.asyncio.session import AsyncSession
 
 from sparkth.core.models.whitelist import WhitelistedEmail
+from sparkth.lib.i18n import _
 from sparkth.lib.log import get_logger
 from sparkth.services.whitelist.exceptions import (
     InvalidWhitelistValue,
@@ -39,13 +40,13 @@ async def add_entry(
                 or domain_part.endswith(".")
                 or ".." in domain_part
             ):
-                raise InvalidWhitelistValue(f"Invalid domain format: {value}")
+                raise InvalidWhitelistValue(_("Invalid domain format: {value}").format(value=value))
             entry_type = "domain"
         else:
             try:
                 _EmailValidator(email=normalized)
             except ValidationError as exc:
-                raise InvalidWhitelistValue(f"Invalid email format: {value}") from exc
+                raise InvalidWhitelistValue(_("Invalid email format: {value}").format(value=value)) from exc
             entry_type = "email"
 
         # No pre-check SELECT: the unique index on ``value`` is the single, atomic guard
@@ -68,7 +69,7 @@ async def add_entry(
             existing = await session.exec(select(WhitelistedEmail).where(WhitelistedEmail.value == normalized))
             if existing.one_or_none() is not None:
                 logger.warning("Whitelist insert conflict for value %s: %s", normalized, exc)
-                raise WhitelistEntryAlreadyExists(f"Entry already exists: {normalized}") from exc
+                raise WhitelistEntryAlreadyExists(_("Entry already exists: {value}").format(value=normalized)) from exc
             logger.exception("Unexpected integrity error inserting whitelist value %s", normalized)
             raise
         await session.refresh(entry)
@@ -78,7 +79,7 @@ async def add_entry(
     async def remove_entry(session: AsyncSession, *, entry_id: int) -> None:
         entry = await session.get(WhitelistedEmail, entry_id)
         if entry is None:
-            raise WhitelistEntryNotFound(f"Whitelist entry not found: {entry_id}")
+            raise WhitelistEntryNotFound(_("Whitelist entry not found: {entry_id}").format(entry_id=entry_id))
 
         await session.delete(entry)
         await session.commit()
diff --git a/tests/api/v1/test_auth.py b/tests/api/v1/test_auth.py
index 0f7fa14b..94ec85b0 100644
--- a/tests/api/v1/test_auth.py
+++ b/tests/api/v1/test_auth.py
@@ -7,6 +7,7 @@
 
 from sparkth.core.models.user import User
 from sparkth.core.security import get_password_hash
+from sparkth.lib.testing import AddTranslation
 from sparkth.services.whitelist import WhitelistService
 
 
@@ -156,6 +157,20 @@ async def test_login_non_existent_user(client: AsyncClient) -> None:
     assert response.json() == {"detail": "Incorrect username or password"}
 
 
+async def test_login_failure_detail_follows_the_request_locale(
+    client: AsyncClient, translation_catalog: AddTranslation
+) -> None:
+    translation_catalog("Incorrect username or password", "Usuario o contraseña incorrectos")
+
+    response = await client.post(
+        "/api/v1/auth/login",
+        json={"username": _uniq("nonexistent"), "password": "Sup3rSecret!"},
+        headers={"Accept-Language": "es"},
+    )
+    assert response.status_code == 401
+    assert response.json() == {"detail": "Usuario o contraseña incorrectos"}
+
+
 class TestPasswordComplexity:
     """Server-side enforcement of registration password rules."""
 
diff --git a/tests/api/v1/test_list_user_plugins.py b/tests/api/v1/test_list_user_plugins.py
index 865df358..5136f4b1 100644
--- a/tests/api/v1/test_list_user_plugins.py
+++ b/tests/api/v1/test_list_user_plugins.py
@@ -17,6 +17,7 @@
     SidebarEntry,
 )
 from sparkth.lib.plugins import SparkthPlugin
+from sparkth.lib.testing import AddTranslation
 
 
 def _plugin_entry(plugin_name: str, enabled: bool, config: dict[str, str], is_core: bool) -> dict[str, object]:
@@ -72,6 +73,31 @@ async def test_list_user_plugins_carries_declared_frontend_metadata(
     assert entry["has_frontend"] is True
 
 
+async def test_list_user_plugins_translates_frontend_metadata(
+    client: AsyncClient, current_user: User, session: AsyncSession, translation_catalog: AddTranslation
+) -> None:
+    session.add(Plugin(name="translated", is_core=True, enabled=True))
+    await session.commit()
+
+    plugin = SparkthPlugin("translated")
+    DISPLAY_INFO.add_item(plugin, DisplayInfo("With Frontend", "A plugin that ships a page", icon="sparkles"))
+    SIDEBAR_ENTRIES.add_item(plugin, SidebarEntry("With Frontend", icon="sparkles", order=2))
+
+    translation_catalog("With Frontend", "Con interfaz")
+    translation_catalog("A plugin that ships a page", "Un plugin que incluye una página")
+
+    response = await client.get("/api/v1/user-plugins/", headers={"Accept-Language": "es"})
+    assert response.status_code == 200
+
+    entry = next(item for item in response.json() if item["plugin_name"] == "translated")
+    assert entry["display"] == {
+        "display_name": "Con interfaz",
+        "description": "Un plugin que incluye una página",
+        "icon": "sparkles",
+    }
+    assert entry["sidebar"] == {"label": "Con interfaz", "icon": "sparkles", "order": 2}
+
+
 async def test_list_user_plugins_empty(client: AsyncClient, session: AsyncSession) -> None:
     user = User(name="Test User", username="noplugins", email="empty@example.com", hashed_password="fakehashedpassword")
     session.add(user)
diff --git a/tests/permissions/test_exception_messages.py b/tests/permissions/test_exception_messages.py
new file mode 100644
index 00000000..40d50f98
--- /dev/null
+++ b/tests/permissions/test_exception_messages.py
@@ -0,0 +1,26 @@
+"""Permission domain-exception messages are translated into the active locale.
+
+The exceptions are raised during request handling, so the message template is
+translated at raise time; all classes share the same marking pattern, exercised
+here through one formatted and one already-covered plain representative.
+"""
+
+from sparkth.core.i18n import locale_context
+from sparkth.core.permissions.exceptions import RoleAlreadyExists, RoleNotFound
+from sparkth.lib.testing import AddTranslation
+
+
+def test_role_not_found_translates_the_message_template(translation_catalog: AddTranslation) -> None:
+    translation_catalog("Role not found: {role_name}", "Rol no encontrado: {role_name}")
+    with locale_context("es"):
+        assert str(RoleNotFound("editor")) == "Rol no encontrado: editor"
+
+
+def test_role_already_exists_translates_the_message_template(translation_catalog: AddTranslation) -> None:
+    translation_catalog("Role already exists: {name}", "El rol ya existe: {name}")
+    with locale_context("es"):
+        assert str(RoleAlreadyExists("editor")) == "El rol ya existe: editor"
+
+
+def test_messages_fall_back_to_english_outside_a_request_locale() -> None:
+    assert str(RoleNotFound("editor")) == "Role not found: editor"
diff --git a/tests/services/test_email_verification.py b/tests/services/test_email_verification.py
index 5c40a05f..ef2fae03 100644
--- a/tests/services/test_email_verification.py
+++ b/tests/services/test_email_verification.py
@@ -8,8 +8,10 @@
 from sqlmodel import select
 from sqlmodel.ext.asyncio.session import AsyncSession
 
+from sparkth.core.i18n import locale_context
 from sparkth.core.models.email_verification import EmailVerificationToken
 from sparkth.core.models.user import User
+from sparkth.lib.testing import AddTranslation
 from sparkth.services import email_verification as svc
 from sparkth.services.email_verification import (
     EmailVerificationService,
@@ -154,6 +156,24 @@ async def test_calls_send_email_with_link_and_name(self, monkeypatch: pytest.Mon
         assert "https://app.test/verify-email/?token=abc123" in kwargs["html_body"]
         assert "24" in kwargs["text_body"]
 
+    async def test_email_content_follows_the_active_locale(
+        self, monkeypatch: pytest.MonkeyPatch, translation_catalog: AddTranslation
+    ) -> None:
+        monkeypatch.setattr(svc.settings, "FRONTEND_BASE_URL", "https://app.test")
+        mock = AsyncMock()
+        monkeypatch.setattr(svc, "send_email", mock)
+        translation_catalog("Confirm your Sparkth account", "Confirma tu cuenta de Sparkth")
+        translation_catalog("Click the link below to confirm your email address:", "Haz clic para confirmar tu correo:")
+
+        with locale_context("es"):
+            await svc.send_verification_email(to="alice@example.com", name="Alice", raw_token="abc123")
+
+        assert mock.await_args is not None
+        kwargs = mock.await_args.kwargs
+        assert kwargs["subject"] == "Confirma tu cuenta de Sparkth"
+        assert "Haz clic para confirmar tu correo:" in kwargs["text_body"]
+        assert "Haz clic para confirmar tu correo:" in kwargs["html_body"]
+
     async def test_strips_trailing_slash_in_frontend_base_url(self, monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.setattr(svc.settings, "FRONTEND_BASE_URL", "https://app.test/")
         mock = AsyncMock()