One of the beat ways to tell really peoficient foreign speaker of, in my case, German isn't the accent (as some people don't have one anymore) but a too perfect Grammar usage, either in writing (less obvious) or speaking (clear as day). I yried one of the German grammar tests of my son, and failed miserable, despite being a native speaker. My wife isn't, and she's so much better in German grammar then I am.

The gist: Don't worry too mich, just use the language. Most people are than delighted to meet a foreigner trying to speak their language.

You might be worrying a bit too much about what others think. I am English and if someone is, say, Russian, and writes to me in English, I don't care if their grammar is a little poor. The great thing about English is you can understand even when most of the words are in the wrong order :-)

Also, most English people don't know much about grammar anyway and lots of people still confuse things like there/their/they're; are/our; its/it's etc.