As far as I know, UI language support is maintained with an addon …
“Manage UI Translations”.
This module creates a tab, with title “I18n Update Translation”.
And an error message …
Could not init language data! Please edit the pref… UI Translate addon
Nibbling around on the prefs, to get files like “src/blender-git/blender/release/datafiles/locale/po/de.po” discovered by the addon. After messing around with the prefs, still same error.
Do you know status of Blender internationalization?