Fix app header language flag doesn't set the user preference in BE

[daniele-mng]

What

• Added functionality to save the selected language in the backend when clicking the flag in the app header.
  The language change is now persistent across sessions.

Why

• Previously, clicking the flag in the app header would change the language used in the application, but this change was not saved in the user settings. As a result, the language preference was not retained across sessions. This update ensures that the language preference is saved and persists.

References

GEA-673

Checklist

• Tests

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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[github-actions]

Conventional Commits Report

Type	Number
Bug Fixes	2

🚀 Conventional commits found.