It causes hesitations in my English as sometimes the French word or phrase springs into my mind first.
Just occasionally I use a French word as if it were an English one when in fact that word doesn't exist, or isn't used like that in English.
I think WB picked me up on that in a post here not long ago...
The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible.
- Bertrand Russell