Eh, Switzerland has among the highest immigrant rates in the world and 4 different language regions... it's federally organsied because it's diverse, not the other wa round.
Yeah I didn't mean to single out Switzerland as cheating or anything, but the federalization keeps the groups clearly distinct. There are technically four ethnic groups that make up the ethnicity 'Dutch' in the Netherlands (actually there are many more in the Kingdom of the Netherlands, but I take it they mean Netherlands proper). Frisian, Lower Saxon, Lower Franconian and Middle Franconian. If the Netherlands would have formed as a federalized state according to ethnic groups (and not united provinces later coalescing into one nation state), people would likely still identify themselves Frisian, Saxon, etc, instead of Dutch. That the diversity rate here is lower than the immigration rate, furthermore means that people self identify as Dutch even when it's not technically their ethnicity.
A similar thing probably happens in the US. Ethnically, the WASP Americans are arguably as ethnically diverse as Western Europe. But whereas we would consider them distinct in Europe, they'd call themselves "American" in the USA, and be considered a single group.