When creating a new field the field name is generated by the field label. When entering special chars like ä ö ü (like in German and other languages) those get transformed to a o u – how it should be. But when entering a ß – like in Größe, which is the German word for “size” (not too uncommon as a field label I would say) it stays as a ß. It should be transformed to ss.
I am not sure if this is really a bug but it could lead to problems.
There may not be a built-in function in PHP to convert special characters / remove diacritics as far as I know, so implementing this might actually require searching for some functions or libraries on the net that have a complete-ish conversion table for common languages.
At any rate, the fields should work even with special chracters.