I also stumbled on this during development of my Qt GUI. First I made it use the standard number parsing functions of Qt, which take the locale into account. Then I noticed that the official client explicitly defaults to US number format (which makes sense for uniformity) so simply I took that over.
Additionally, like mentioned by some one in the german subforum, we could split the entry field into 2 entry fields with a fixed , or . between them.
I've thought about this as well. Simply force the '.' in the middle and regard it as two fields, one right aligned and the other left-aligned. Pressing the '.' while in the first field will jump to the second field.
I'm going to implement this and see whether it is useful.