Gerry, residing in Germany, I often caption images with German ä, ö, ü and have never encountered LR to freeze. The software suddenly jumps to some other function but never freezes. Unfortunately I haven't been able to find exactly which umlaut letter causes problems - it might be a combination of two letters typed is sequence. I need to look at the keyboard while typing and later notice that something else has happened.
I've found it quite aggravating to use LR with a German keyboard and also trying to find a common language with which to caption images (English or German?). It haven't been able to find the German equivalent for the backslash key. A number of times I've listened to Michael, Jeff, Seth, Julieanne and other speak about using keyboard shortcuts which simply aren't available (at least I haven't found them!) using that darned backslash key.
Hopefully someone will offer you a functioning solution to your problem which may also offer me some aid.
By the way, do you caption your images in one or more languages? And how do you distinguish between them? I'd like to caption one in black, the other in dark blue or use cursive font to distinguish - some form which is readably visible. Sadly, I haven't found any solution which LR permits.