Umlauts in lyrics

Ah, English keyboard, sorry.
Try F2 and pick them from the palette
Or copy/paste them (from above)

How did you write it in your question? Isn't that how it works in MuseScore?

And writing an 'ä', 'ö', 'ü' as 'ae', 'oe', 'ue' is also accepted by Germans.

Another possibility: Use "Extras -> Copy lyrics to clipboard", paste the lyrics into a text document (Notepad), attach it here and have it corrected by someone with a German keyboard (e.g. me :-)).

Or use this one: Ä Ö Ü and ä ö ü, sometimes ß is also necessary. Save the letters in a simple text document so that you can use them again and again as a copy template.

If you're an English speaker, using Windows, and going to be typing text in German with any regularity, may I suggest you add "English (United States)--International" as one of your keyboard options:

Settings>>Time & language>>Language & region>>Options>>Installed Keyboards>>Add a keyboard.

It will allow you to type all the characters and diacritics (umlauts, etc.) in German, as well as French, Italian, Spanish, and the Nordic languages. You don't have to use it all the time, but you can toggle back and forth between it and your usual keyboard.

Here's a link to give some tips: https://slcr.wsu.edu/help-pages/microsoft-keyboards-us-international/

US-INTL won't provide some characters in Polish, Hungarian, Czech, etc., but there is a keyboard layout called Alt-Latin 2019, which will--however, it is not native to Windows and will require downloading.

And even Alt-Latin won't handle Turkish (that dotless "i").