Then I am not sure if I understand the original sentence: is he the teacher of the subject?
How is He teaches Russian different from He is teaching Russian? I put the latter and it wasn't accepted.
To say someone is a teacher of Russian describes their profession or hobby, but doesn't indicate they are practicing at that very moment.
Languages just prefer different prepositions in different cases. It's one of the things that has to be learned by heart.
I feel you could use the proposition "in" in English if you said "He lectures in Russian" and that would imply he teaches the Russian language, but you couldn't say he teaches in Russian to mean the same thing.
Do I have this right?
Han är lärare i ryska = He is a teacher of the Russian language
Han undervisar i ryska = He teaches the Russian language
Han undervisar på ryska = He teaches (something), and uses Russian to do so
English is not my native language but I feel you could also say "in Russian" instead of "of Russian".