Most of Europe's languages fall into one of the three categories Slavic, Germanic or Romance. Your challenge is to find a way to replace one of these language families with another one, that does not mean it has to be eliminated completely it just has to be reduced to one of Europe's smaller ones, like Baltic, Celtic or Finno-Ugric. Bonus points of your language family of choice isn't an indo-european one.
I think the easiest language family to replace is Romance, since it spread because of the Roman Empire. If you remove the Roman Empire you could easily see most of Westernmost and South-Western Europe speak Celtic languages instead, with Romance being limited to Italy and the Dalmatian coast.
I think the easiest language family to replace is Romance, since it spread because of the Roman Empire. If you remove the Roman Empire you could easily see most of Westernmost and South-Western Europe speak Celtic languages instead, with Romance being limited to Italy and the Dalmatian coast.