My guess is that Kylo was able to translate the dagger, that’s how he knew. But we’re never told that, and it’s never even hinted at. This is a pretty cool way to show how he knows.
Yes that’s canon. I’d say it’s at least slightly hinted at since they were able to figure out so much about the Aki-Aki necklace so easily. So a Sith relic like that would be a cakewalk for whatever they’ve got in their databases (especially since they aren’t bound by laws passed by the Republic).