German style guide for capitalization rules of UI elements

I want to improve the text consistency in our software, because over time some things got messed up and we have different styles for example in captalization of UI elements (controls). I find a lot of style guides for English capitalization rules (Microsoft, IBM, Apple, ...) but not for German.

Do the english rules for title and sentence case also apply to German?

Can you name me some sources for german style guides?