You'd be surprised. Your written english is better than many Americans. I don't think letting people know that english isn't your first language is necessary. You're great. Americans seem to have greater difficulty with punctuation, which is what makes for more difficult reading than just an occasional misconjugated verb, or improper pronoun.
that is because when you learn english first, you are learning on the fly, so you do not get to learn all of the rules, so than when it is time to learn the rules of the language, people get bored and just ignore what is going on in the classroom.... when you are learning as a second, third, or later language, than they are learning the rules first, than how to speek, that and they actually care about learning because they want the knowledge.
Actually you are completely wrong when it comes to English as your secondary or third or whatever language.. Most people in my country learn the language at a very early age thanks to the internet, the television (we have subtitles here, no dubs), certain magazines (game magazines for example),..
I think the main reason why we learn english so fast is, that we "constantly" hear english speaking on the television and in the meanwhile see the translation below.
Its a very effective way of learning a language. Also the internet has had a serious influence: online gaming for example.