So if they used QuestyCaptcha on a multilingual site like Commons they could only have the questions in the site language? I guess that Commons would choose to go with a different type of Captcha, such as a sum, which most scripts and language could handle.
Sure, QuestyCaptcha is not a viable solution for wikis with many thousands or millions users. Wikimedia projects use FancyCaptcha (which is broken as well, but has at least an appearance of correct functioning).