While the thesis work may be in English (depending on your field), that doesn’t magically change the country you’re living in. France requires French, unless your plan is to just come for your PhD, primarily stay in an expat bubble, and leave after your thesis. Everything administrative, plus integration outside of Anglophone bubbles, expects you to be able to speak French. Sure, some people manage without — but it’s highly dependent on their situation, means, and location. And it’s not fun on the long term, having seen some people try.