At the same time I very often end up misinterpreting what my English colleagues are telling me because they'll use words that mean X thinking they mean Y or they'll simply write a phrase in a confusing way that makes sense when spoken, but won't when it's written.

What many unilingual people don't realize (and I'm sorry but... Anglophones tend to be in that category more than most) is that if you're bilingual, the second language you learn is taught to you using the "official rules" from the beginning, you don't spend years building bad habits that need to be forgotten in order to pass an exam. What that means is that you'll often realize that a person who knows English as their second language will be better at writing it and spotting mistakes than native speakers.

One such example is native speakers writing "could of", something that non native speakers know right away makes no sense... Or my personal pet peeve, "I could care less", damn this one triggers me!