Resolve "Change app language does not work"

Closes #1764 (closed)

Test instructions:

• Precondition: be aware of your device settings. Relevant settings are:
  • language: System settings → System → Languages & input → Languages
  • device theme (API 29+): System settings → Display → Advanced → Device theme
    • if your device is lower than API 29, assume device theme is light
    • if your device theme is API 29+, set your device theme to light
• Test that selecting a different language works
  • Open briar, navigate to settings
  • Make sure theme is set to "Light" or "System default"
  • Choose a language other than the system's default language that you're familiar with
  • restart briar by confirming the dialog
  • (A) check that briar is properly translated into the selected language
    • on every screen encountered, it's not necessary to check all items individually for correctness of translation, it's a matter of scanning the screen and making sure all things that appear are translated into the selected language
    • open a few activities/screens. Pay special attention to :
      • the activity titles at the top (Contacts, Settings, etc)
      • the nav drawer content
      • the items in the settings screen
      • the menu items in any of the conversations
• Test that selecting a different language works with a different theme selected in briar
  • Open briar, navigate to settings
  • Reset language to System default (restarting briar)
  • Open briar, navigate to settings
  • Select theme "Dark"
  • Choose a language other than the system's default language that you're familiar with
  • Repeat steps (A) above