TBH English is a pretty easy language to learn. Grammatically it's incredibly simple - no gender, verb conjugation is really basic, no declinations, etc. Most learners find pronunciation to be the hardest part. Even then, though, English pronunciation varies a hell of a lot between regions, making it significantly more forgiving than, say, French.
It's interesting because the mistakes that non-natives make tend to have little in common with the mistakes that natives make; it would be incredibly rare for a non-native to produce "could of" in writing, for example.
I taught English as a foreign language for 10 years and subsequently I don't really care about most mistakes people make. The only ones that really irritate me are the ones that are an attempt to sound more intelligent/sophisticated - the misuse of "and I" when it should be "and me", "whom" used when it should be "who" and, worst of all, the terrible abuse of reflexive pronouns in phrases like "please get in touch with myself". That's just painful!