Improve German localization [Part 1]

[H0mebrewL0ver]

As discussed in https://github.com/V10lator/NUSspli/pull/331#issuecomment-2243396848 I split the improvements into multiple PRs. Part 1 contains:

• capitalization and rewording of words where necessary
• grammar improvements for some buttons
• fixes for some minor mistakes
• minor adjustment for Continue. I think Fortfahren fits better