For comparison, here are some German cities with a similar number of inhabitants as Leibnitz:
Rosbach v. d. Höhe
Steinheim an der Murr
LT knows some of them and others not. Unfortunately there doesn't seem to be a systematic approach.
Another idea: the spell checker should not just provide a suggestion but brielfy explain it, like: "Leibnitz [Stadt in der Steiermark]".