ST Forum is not unique in this shortcoming.
Xenforo forum software package is one of several notables not having support for characters entered via non-English (US/UK) keyboards.
French Keyboard : ALT GR + [ + u - This should be umlauted u but it turns out to be: ü
German Keyboard : [ + u - This should be umlauted u but turns out to be: ü
I realize that this is not priority as English is the lingua franca for IT stuff.
But it is available within Stack Exchange forums and perhaps it might not be too difficult to offer it.