This is nuts, if i learned Mandarin, i would probably move to Taiwan, where the CoL is almost 1/4th of where I am right now. I know some other languages, but loss the will to study anymore lol.
I'm a foreigner who's live in Taiwan over the past four years. You can move here tomorrow if you want :D. You can survive with English (though your life will be easier with Mandarin), and there are plenty of English teachers who never learned Mandarin who've lived here for 10+ years.
Highly recommend living here. I met my wife here. Life's chill. Nobody steals. If you've made at least $60k in salary at least once in the last three years you can apply for a Taiwan Gold Card (kinda like a Taiwanese O1 Visa) and come live/work here very easily.
The main reason I continue to learn Mandarin is because my mother in law speaks zero English, so it just makes everyone's live's more fun and pleasant if I speak some Mandarin. :D
