I think there would be enough 5 Letter words without umlaut. And even if they are really needed it could be done this way:
ä=ae
ö=oe
ü=ue
Do you think translation of your words would be possible, or better choose completly new ones?
Translation from English is unsuitable since there are thousands of words and many wouldn't be 5 letters once translated. See "answers.txt" and "full.txt"
https://github.com/bbbbbr/gb-wordyl/tree/main/compress
Better to find an already compiled German word list (perhaps even Word list) and then filter that to 5 letter words (and as you suggest, exclude ones with special chars is sufficient).
Then split into a set of most common words (answer list) and remaining words. Sometimes that sorting is already available somewhere. And perhaps look through answer list to remove the more "offensive" words.
I put German word lists reduced to 5 letters and English A-Z in a zip file here:
https://github.com/bbbbbr/gb-wordyl/issues/1
If you're interested, you can take a look to see if they are suitable.
If they are, and you are up for it, it would be nice if you might skim through and remove typically offensive words from the shorter "answers" list (this shorter list is where it chooses the secret word for each round).
I built a ROM with the German answer list and dictionary then added it to that ticket (you're also welcome to comment there if you'd like). I did some brief testing and it seems to work as expected.
If you want to give it a try that would be great.
https://github.com/bbbbbr/gb-wordyl/issues/1
If it looks good, maybe I'll add an intro popup message indicating the language and explaining the limitations.