I guess this is based on the Accept-Language header of your browser (which you can control), not guessed from your IP (which is a terrible thing to do): since m

I guess this is based on the Accept-Language header of your browser (which you can control), not guessed from your IP (which is a terrible thing to do): since my OS is in English even though I'm in France, I get to have the English wikipedia with !w and use !wfr for local stuff. In fact I did not know about the behaviour you describe because of that.