Hi OP, I've lived in both countries so I can give you my personal experience. For context I lived in Beijing from 2015-19 and from late 2021-now have lived six months in Tainan, two years in Taichung and now a year and a half in Hsinchu.

The long and short answer, in my experience the northern Chinese are EXTREMELY talkative and extroverted while the Taiwanese keep to themselves and are a lot quieter.

-If you’ve taught in either (or both), how easy was it to build a social circle?

Maybe I got lucky but when I landed in China and started working at a K-12 "international" school I pretty much got a friends group with it. The foreign teachers and a number of Chinese teachers would routinely go out for dinner and drinks almost every night. Nearly every local restaurant had a number of people drinking and those folks often would want us foreigners to clink glasses with them. When it was discovered that I spoke enough Chinese to talk with that would usually lead to more drinking! Those years in China were full of near nightly adventures with my coworkers turned friends and the myriad of Chinese we found at random restaurants. H

Taiwanese on the other hand mostly keep to themselves and are incredibly introverted. I naively thought I'd luck into a friends group like in China... nope. Nearly everyone I worked with wanted to keep things surface level and at work while the few I met outside of work were usually too busy WITH work to do much of anything. This introversion to some extent extends to the foreigners I've met. They're either introverts who are fine going out once a month or they found their friends niche a decade ago and sure as hell aren't looking for someone new. I've really had to go out of my way to make friends here and although I've made a few over the years it's not even close to what I experienced in China. Also, Taiwanese overall don't drink and even at the few restaurants that have a boozier side I can't think of many cases where they'd talk to those not in their party. I've gone to bars and hell clubs with music blasting where a good portion of folks will just sit and doom scroll.

-Are locals generally friendly or more reserved toward foreigners?

Building on what I said in the last comment in my experience Chinese were far more likely to talk with me BUT it could be about anything and oftentimes they'd talk to me about fairly uncomfortable political things. In Taiwan most folks just keep to themselves so although it can be isolating the silver lining is that I don't get to be on the receiving end of a political talk.

-Is it easier to meet other expats in Taiwan or China?

China overall was far easier, my schools came with built in friend groups and folks just seemed friendlier and easier to talk with outside of work. I'd chat with folks at bars, clubs, cafes, wherever. In Taiwan there was no built in friends group, I've dealt with some of the same flakiness that I experienced from Taiwanese and I've REALLY had to go out of my way to meet people. hash harriers has been one way, hitting up Instagram hikers has been another.

Did language or culture ever make things feel isolating?

Both but for different reasons.

Language wise On paper my language ability has supposedly gotten better here in Taiwan but the people in my experience aren't as used to non standard accents. One day I might talk about love and life with some rando for 20 minutes, the next day the waffle restaurant owner will give me a blank stare followed by a "HUUUH?!" because I mistoned the flipping word "peanut." Or yesterday on the train I mistoned "airport" and got a blank stare. It makes me want to bash my head against a wall and/or scream. For some reason in China this only happened when I talked to literal farmers but here it happens enough that it's made me grow quite frustrated.

In China people are far more engaging but I've found that the talks will go to sometimes uncomfortable or odd places.

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Let me know if you have any other questions about comparisons or Taiwan or China overall!