When I create a revocation certificate and enter an umlaut like 'ä' in the
reason, this gets masked to \x84. This seems to be incorrect because this cannot
be translated to an utf8 character without knowing in which encoding the
revocation certificate has been created. This is even shown like that in the
next output line after entering the reason, where i have to confirm the reason.

Output in my console (cp 850, default font) after calling gpg --gen-revoke <keyID>:
Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:

0 = No reason specified
1 = Key has been compromised
2 = Key is superseded
3 = Key is no longer used
Q = Cancel

(Probably you want to select 1 here)
Your decision?
Enter an optional description; end it with an empty line:

Täst

Reason for revocation: Key has been compromised
T\x84st
Is this okay? (y/N)