My native tongue is not American. In fact some accuse me of using ellipsis too much, both a Mexican and British Literary tools. I learned to use them young, they became part of who I am.
But I became a writer (serious one) in American. Yes, I have short stories I wrote in HS and poetry, in Spanish. I have tried to write Spanish, but in a literary way, I don't think that way. What I still have, is a penchant for long sentences. Both also sins of Queens English and Spanish.
I know there is a market out there in Spanish for sci fi and fantasy. The genre is still in baby stages, and most are translations from both the English and American markets. Mexico has a tradition of fantastic realism that one of these days I will try in English, so it should not be hard to produce good quality work within that tradition in Mexico. But I just simply don't think that way.
I admit, recently I started to read Mexican literature in it's native tongue. Partly it is to recover a facility of language that is in some ways no longer there. Writing in it seems daunting.