If you are from the US and stay there, English is the global Lingua Franca, the local Lingua Franca, the language of the country you live in and your mother tongue, and thus you will likely never learn a second language to fluency levels.

Well, sorta.

In my experience with British colleagues when living in The Netherlands (were you can definitelly get away with speaking only English), whilst some of them never really became fluent in Dutch, others would become fluent in it.

You see, even with English being a lingua franca, many if not most of the locals (how many depends on the country and even area of the country - for example you’re better of speaking broken German with the locals in Berlin than English) are actually more comfortable if you speak their language, which make your life easier. Also the authorities will often only communicated in the local language (in The Netherlands the central authorities would actually send you documents in English, but for example the local city hall did everything in Dutch).

That said, if you’re an English speaker you can definitelly get away with not learning another language even when living elsewhere in Europe plus I’ve observed that in the early stages of learning the local language often when a native English speaker tried to speak in the local language the locals would switch to English, which for me (a native Portuguse speaker) was less likely, probably because the locals could tell from a person’s accent if they came from an English-speaking country hence they for sure knew English whilst with me even if they recognized my accent they couldn’t be sure that I spoke English.