The UK is a country, but it doesn't have its own language per say. Depending on what region of the UK you might say it with an accent or with slightly different vernacular, but on the whole, Merry Christmas would be fine.
There are differences between American and British English but we both say Merry Christmas.
Just as in your question !They tend to say "Happy Christmas" instead of "Merry".Merry Christmas
You say ' Merry Christmas' in the UK.
merry christmas
"Merry Christmas"
Merry christmas
merry christmas
Merry Christmas
merry Christmas= Christmas aashamsakal
Hanukkah is a holiday, not a language. You cannot say "Merry Christmas in Hanukkah. That's like asking: "How do you say Merry Christmas in Easter" I think he means how do you say merry Christmas in Hebrew
Merry Christmas
Merry Christmas
Merry Christmas