Shaklin
Note, on monkeygames.uz there are many games of Sierra in German language.
I just yesterday read about the structure of vocab.000. I had never dealt with this issue before. There's no way to add umlauts to the vocab.000.
You have four ways.
1. Translate messages only. Typing from the keyboard to leave in English.
"open the door"
not öffnen (aufmacht) die Tür
2. Translate messages and add to vocab.000:
oeffnen, ueber, dass... etc.
In read.me tell everyone how to enter commands.
3. More complicated. Edit the font.
Replace in font
: -> Ö
; -> ö
" -> Ä
' -> ä
{ -> Ü
[ -> ü
_ -> ß
$ -> :
% -> ;
^ -> "
& -> '
In text messages replace all occurrences of a characters : on $, ; on %, " on ^, ' on &.
In vocab.000 add necessary words:
[ber (instead über)
;ffnen (instead öffnen)
T[r
da_
G'stezimmer
etc.
4. More more complicated.

I'm not sure that will work.
Get sciv.exe from SQ3 German or Larry3 French or Larry3 German or KQ1 (SCI remake).
These games were written with SCI01 interpreter.
Use vocab.900 instead vocab.000.
Edit vocab.910 normaly.