One of my favorite examples of a multi-ethnic state in Europe (kind of biased since I have some ancestry from there) is the Polish-Lithuanian commonwealth. That lasted a good long while with a mix of Balts, West Slavs, and East Slavs.
But the most ridiculous thing is that multiculturalism killed it. It was partitioned by foreign imperial powers, eg. split between Russia and Germanic powers.
And that, arguably, was one of the causes of its eventual demise - e.g. would Ukraine have rebelled if they didn't perceive themselves as second-class citizens in Rzeczpospolita? And if it didn't, would Russia be able to eventually take over half of Poland?
It might shape your view a bit
Second, languages had nothing to do with it. There was an idea that sacrificing a part of Czechoslovakia will be enough to keep Hitler at bay (https://en.wikipedia.org/wiki/Munich_Agreement).
Languages had everything to do with it. "Bohemia became a part of Czechoslovakia, defying claims of the German-speaking inhabitants that regions with German-speaking majority should be included in the Republic of German-Austria."
The German-speaking inhabitants, that is a whole different story.
English is spoken pretty much everywhere. What's your point?