i don't understand why is the preposition "i" used in this sentence, if it means "Who will teach German?". How can i say "Who will teach in German?". I mean what preposition should i use in the second question instead of "i" ?
You use ”i” with the verb ”undervisa”, that’s all. If she would teach in German, it would be ”på tyska”.
Well, there is lära dig (teach you) but notice that there's an object there (dig), in contrast to the sentence above, which does not say who is being taught and is just talking about teaching in general. I think the non-reflexive lära ut is roughly synonymous with undervisa, but I'm not sure if there is any subtle difference in meaning between them.
"Skall" is largely archaic, and thus rarely used. You might see it in government orders or so.