Anecdotally, not a single American I've talked to who was on the fence about moving to Europe was deterred by not knowing the native language well enough. Whether overly optimistic or not, the prospect of spending a few months learning well enough to barely get by and spending more time later to become more adept never amounted to even a footnote in the deciding factors.

It's irrelevant, you can get by with English for the basics in most parts of Europe unless you move to small villages. If that weren't true then the EU's obsession with freedom of movement would be pointless, because English is the only language that is universally taught in Europe.