Pages that start with an umlaut (ä, ö, ü) character should be placed in the a, o, u categories in the index. They are currently visible only when "All" categories" are selected.
"ä" is put after "a" but before "b". "ß" on the other hand is treated like "ss". So it should be Godel, Gödel, Goedeke, Goethe, gotisch.
It is non-trivial to properly do international sorting, but I think Java already has methods to handle this (Collator?).