I noticed earlier in this skill a sentence that said Tysk er vanskelig. Can tung and vanskelig be used interchangeably this way?
Tysk er vanskelig*
in this way, yes, and some other ways.
No Norwegian would use that sentence, I'm afraid. It's either "Er tyrkisk et vanskelig språk" or "Er tyrkisk tungt/vanskelig å lære", which means "Is Turkish hard to learn".