Sylvan is swedish american but she doesn't know a lick of swedish (mainly because she was born and raised in a place where everyone speaks mainly english) and she's too busy focusing on becoming a fashion designer to actually learn other languages, plus it's already hard for her to understand school subjects so you would think learning a new language is 100x harder for her
Just like Sylvan, Hiro is also born and raised in a place where everyone mainly speaks english so he only knew english in step 1, also I would've liked to think that he's not really interested in learning a language unless he's really interested in it. Though in step 2 he learned german for Tammy (another reasson is that he's really curious on what Tammy says in german sometimes)