This is a huge hypothetical scenario, I'd caution any experiment with an N=1 but this is an N=0 example. Anecdotally I don't think my French is worse off becau

This is a huge hypothetical scenario, I'd caution any experiment with an N=1 but this is an N=0 example. Anecdotally I don't think my French is worse off because I learned Esperanto first, (nor any of the other languages I learned). If anything, learning more languages makes me more aware of the importance of pronunciation as a meta-learning aspect of languages. It's not like those French roots aren't completely mangled by English learners anyways, words like "-ion" in "situation" and "conversation". But you learn just like any other learner.