For me this is not only limited to smut. I used to read fics in French when I was 16 because the internet was starting and I didn't follow many English sites. But as of now I would absolutely be unable to read a fic in French. Something would seem off, smut or not.
But I may not be a good specimen for this kind of study lol- I live through the day in English. My phone, laptop, video games, I set everything to English. I read books in English, I talk to myself in English, and sometimes I mix up words when I speak French (my husband never misses an opportunity to make fun of me ahem)
So... I don't know. But this is an interesting topic maybe we should split this one to another place? (I'm on my phone in the car so I can't do much - I'm actually impressed that I managed to write this much without getting p*ssed at the phone keyboard lol)