Lance speaks a little bit of a lot of languages, although most of what he knows is fairly archaic now.
Vaivait, his planet, was a resupplying hub for several intergalactic trade routes, and it was useful to most people on the planet to know at least a few words from the most common languages that were spoken along the trade routes.
Lance is fluent in Altean, of course, and also Vaitie, which was the most common native language spoken on his home world, although there were some other common native languages he knew a little bit of as well.
He’s conversational in Galra, although it’s the middle english equivalent of Galra, since 10,000 years will do that to a language. He knows it well enough that he can mostly pick up on what they’re saying, although it all sounds a bit odd to him.
He doesn’t let on that he knows Galra for a while, particularly after they start interacting with the Blades of Marmora, because he doesn’t really trust them at first (not because they’re Galra, just because they’re weird, and Keith came back from their first meeting with them a mess), and he likes being able to eavesdrop on them.
This lasts until one of the Blades says something rude about someone on the team, and Lance tells him off for it. Kolivan is so amused, he doesn’t even get upset about Lance hiding it.