complete translations

[scheidtdav]

We need to go through the entire codebase and add missing translations where necessary. Some ui component simply do not respond to language changes via i18next and/or simply don't use the t function.

We should at least have german and english translations everywhere.

[timber-they]

I started this but might not manage to finish; it should be easy to pick up the work in that case. I'm working on the branch feat/complete-translations.

[timber-they]

I have finished most translations except for some error / info messages where I couldn't figure out how to use the translations (I added a TODO). I was actually mistaken and my last working day is already today, so I won't be able to continue working on this. I think it can already be merged, since it's already an improvement to the current state, but someone could also continue working on the branch.

[scheidtdav]

@timber-they Thank you for the preparing this far! I will add the missing pieces, no worries :-)