Add language selection

[florianschwanz]

Is your feature request related to a problem? Please describe.
The user shall be able to selected their preferred language on the start screen.
This shall affect the stacks they see.

Describe the solution you'd like
Display a language selection screen when the app is started for the first time.
Store the selected language in PouchDB for subsequent sessions.
Add a menu item that opens the language selection screen.

Describe alternatives you've considered
tbd

Additional context
Currently the language is set automatically based on the browser language.