It's like my second mother tongue, so somewhere between fluent and mother tongue.
I am not that sophisticated in it like e.g. masterclass and I don't like to write long essays here on MTF, but I understand almost everything which doesn't involve some heavy slang both in British and American English.
I came to this forum expecting a lot of posters to not be as proficient in English (given the international identities of posters). How wrong I was. A Polish woman in Nathaliia writes almost as well as the girl next door. So it seems most of the folk here are pretty well educated and can construct more than simply coherent sentences, and with little in the way of noticeable 'gaps'. I'm guessing there is strong proficiency in writing skills and reading across the board, but perhaps not so much in speaking (though that is quite natural).
It still amazes me I get to talk to Swedes, Hungarians, Chinese, Romanians, Serbians, French, Brazilians etc. all on one forum in a language we all understand.
Actually I do remember explaining to you the meaning of 'piggy back' - which you considered an offensive phrase at the time