I didn't know 'wanna' was an informal contraction in Swedish, too! Honestly, I'm kind of glad Duo doesn't really delve too deep into slang and colloquialisms, I'm sure anyone trying to learn English's head would spin at the multitude of them we have.
kinda sorta shoulda coulda woulda gotta gonna wanna dunno
In all of those except for "dunno" all you are doing is removing "of", "have", or "to" and ending it with an "a". It's really quite simple and easy to learn I would say. English in general is not that hard to learn compared to many other languages from what I have heard from all of the people I have talked to so far also.