Barcelona feels like Spain? Frankfurt and Berlin feeling generally like the same country?

I can agree that London is the most international city of Europe -at least right now, probably not in the close future-, but your descriptions of the other cities feel very off.

40% of Frankfurt residents have non-German passports. It definitely doesn't feel like the rest of Germany. There are tons of Turks, Italians, Croatians in particular, along with workers from all the EU member states. You can theoretically live and work without speaking any German, although obviously not having it makes life much more tricky.