[DE] Use more common words

[The-Last-Cookie]

Proposal to use more common words on the main page.

A bit unsure if we should use Song-Setup instead of Song-Einstellung for the beatmap editor tab from now on.

Self-check

• [x] The changes are tested against the contribution checklist
• [ ] (translations only) The changes are reviewed on GitHub by a fluent speaker

[FamousPig]

I should maybe read the description first :P

I would be fine with using Song-Setup. I think it better confers it's function as the screen you usually go through first before creating the actual map, as opposed to something like "song settings".