The following map shows how to say (or rather write) the equivalent of “Merry Christmas” in European languages (not all minority languages are included this time because the map would be too cluttered otherwise), however in our case we will focus on Slavic languages. The coloring corresponds to etymological relations between the translations of the word Christmas (i.e. not to language families and not to relations between other parts of the phrase).
This interesting map by Jakub Marian has lead to a few unexpected results. Even though Romanian and Hungarian are completely unrelated languages, the words karácsony and Crăciun come from a common root (either Proto-Slavic *korčiti or Latin creātiōnem).
Something quite unusual happens in Czech and Slovak. The word Vánoce resp. Vianoce is derived from German Weihnachten by retaining the “Weih” part (which comes from an old Germanic expression meaning “holy”) and replacing nachten (“nights”) by the Czech/Slovak translation, “noce”.
However, the word “noce” itself comes from the same Proto-Indo-European root as German “Nachten” (and English “nights”), so the Czech/Slovak and German expressions are essentially etymologically equivalent.
So you can on example see how Slovaks and Czechs changed the saying into a Germinized form, while Hungarians and Romanians have a Slavicized saying for Merry Christmas.