Make Metadata.File default language configurable

[tushabe]

Summary

Make the shared user language of Metadata.File configurable through its OSGi component configuration.

Metadata.File currently hardcodes German in a static field. As a result, Backend installations that use this metadata provider cannot choose another initial UI language, and changing the language affects all provider instances in the JVM.

This change:

• adds a defaultLanguage OSGi setting, defaulting to DE for backwards compatibility
• keeps language state scoped to each Metadata.File instance
• creates the shared user with the configured language during activation
• updates the current shared user immediately when updateUserLanguage() is called
• documents the setting and all currently supported Language enum values
• adds focused tests for configuration, instance isolation, and runtime language updates

Example:

path="/var/opt/openems/metadata.json"
defaultLanguage="EN"

Validation

Validated against the current upstream develop branch with:

./gradlew --no-daemon -Dorg.gradle.vfs.watch=false \
  :io.openems.backend.metadata.file:checkstyleMain \
  :io.openems.backend.metadata.file:checkstyleTest \
  :io.openems.backend.metadata.file:test \
  :io.openems.backend.metadata.file:jar

./gradlew --no-daemon -Dorg.gradle.vfs.watch=false \
  checkstyleAll build resolve --console=plain --warn

Both commands completed successfully.

The same implementation was also built into the Nearly Free Energy pilot Backend, deployed with defaultLanguage="EN", and verified through an actual UI logout/login and a Backend container restart. The UI opened in English and the configured default remained effective after restart.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #3850      +/-   ##
=============================================
+ Coverage      60.63%   60.65%   +0.03%     
  Complexity       105      105              
=============================================
  Files           3496     3496              
  Lines         153207   153212       +5     
  Branches       11691    11691              
=============================================
+ Hits           92887    92921      +34     
+ Misses         56824    56804      -20     
+ Partials        3496     3487       -9     

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