A captcha works because about 95% of all spam comments originate in China, regardless of the type of spam. They are inserted by bots. WordPress stores the IP address for the spam comment in the database. Last year I did an analysis and found the about 95% originated in university cities within China. It did not matter which language was used in the spam, or the nature of the comments.
Given that they mostly originate in university cities in China, it sound like building spam bots is a typical class assignment.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin