Language requirements?

This may be a dumb question, but as a non-pilot enthusiast, I’ve always wondered…

(For this question, let’s say the pilot is American and flies for United… and is flying to Japan.)

Q: “If a pilot wants to fly international routes, is it the Pilot’s responsibility to learn other languages; or is it the responsibility of the destination airport tower to speak the Pilot’s native language?”

Thanks in advance.