You’d use ”om” (about), preferrably with the verb ”handla (om)” (to deal (with); to be (about)”: Presentationen handlar om svenska. or you could also say Presentationen är om svenska.
Perhaps lärare i tyska? (Teacher in German). In that case the language name is a subject and then you use it, but in Swedish you speak on languages, rather than in.
It was "exercises in German" i believe. It was also discussed in the comments that "i tyska" implied that they were exercises in a German language class and "på tyska" meant the excercises were in the German language.