I wonder if you have an affinity for languages that other people don't.
You can definitely write English- I would never have guessed you weren't a native speaker of English!
I've tried to learn other languages and have not been that successful with any of them (and these were Romance languages- not something like Japanese!). And I know people who have lived and worked in a predominantly English-speaking area for years and still are not what I would consider fluent, so maybe what isn't that hard for you is considerably more difficult for others. I think the errors that jar me the most are when it's someone who otherwise is a very good writer and it happens repeatedly so it's unlikely to be a typo. It's the dissonance that gets me, I think, not necessarily the mistake itself.