A city best known for its millionaires and factories churning out everything from buttons to lightbulbs is now better known in China for its unintelligible dialect.

That's according to a post titled "China's Top 10 Most Difficult Dialects" – known as fang yan in Mandarin – that's been trending on Weibo, China's microblogging service. Wenzhou, the free-wheeling commercial metropolis in Zhejiang province, took the crown.

"Even...