If you were going to say "the lesson is ON German," you'd use "om"?
Yes, you would.
We normally do not use this word "leksjon" in daily spech, though.
what would you typically use colloquially?
Leksjon vs time, anyone? Which one is more frequently used?
if I had used i tysk, would it just not make sense because i'd be talking about the physical location of the lesson?