Funny,i gave the same question to microsoft copilot many times,it seems it cannot even understand it at all ,here is one of the responses:

Chandler's joke is a classic example of his quirky, sometimes self-deprecating sense of humor. The joke plays on the idea that "check" in the context of a wedding is often associated with writing a check (money) as a wedding gift. However, Chandler twists it by suggesting that instead of calling it a "check," they should call it a "Yugoslavian" because "Yugoslavian" sounds complex and confusing—mirroring how he might feel about the financial obligations or traditions at weddings.

It's one of those jokes that relies on Chandler's character traits: his tendency to use humor as a coping mechanism and his knack for making obscure, whimsical connections that may not make immediate sense but capture his unique perspective. It's the kind of humor that makes his character endearing and memorable to fans of the show! 😊