Over on Another Fandom, there are several authors whose native language is not English--and it shows.
Joseph Conrad's first language was Polish, but he became a major novelist in English; this is very unusual No matter how good you get in a foreign language, it is almost impossible to become a really fluent writer in it.
I would much rather read a story in good Spanish or Italian than in bad English.
Please, please, if you want your story to be read in another language than your own, write it in your language and get someone to translate it for you.
(This also applies if you want to have bits of another language in your story and you aren't reasonably proficient. It isn't so bad in this fandom, but some writers who want to have bits of Latin in their stories go to a Latin-English dictionary and start throwing words together--it doesn't work that way. Find someone who really knows the language and ask him/her to help you.)