Why do people say China is “homogenous” when people speaking Cantonese can’t understand people speaking Mandarin? It’s because those groups of people have been part of a common polity off and on since the Qin dynasty more than 2,000 years ago.

Similarly, Switzerland has been a thing, in various forms, since French and Italian were considered different dialects of vulgar Latin. The groups that speak the four different languages nonetheless share 800+ years of common political and social history.