Gimli would teach Legolas anyway, and cheerfully toss all the disapproving and the scornful into the trash. And laugh heartily at Legolas’ absolutely AWFUL accent and his inadvertent vocab gaffes (”Ah, no lad – you just said “I hope you have a nice bowel movement”. “Ai, I only meant to wish him a good day!” “Well, perhaps a nice bowel movement means a good day for him, you never know. With a sour face like that, in fact, it seems quite likely.” “Meleth nin, you are not helping me here.” )

I expect Bilbo would be INCREDIBLY eager to learn. He loves languages, after all. He probably wouldn’t push to learn, bc cultural taboo, but I bet he’d store all those little commonly said words away where he can pore over them later. LANGUAGES, OH MY. (To his chagrin, he later discovers that most of them are curse-words. Well, the language most people swear in is their mother-tongue, and Thorin did stub his bare toe…)

Culturally, yeah – I think that society would be a bit divided on the issue. Some would consider it only right and proper that a non-Dwarven spouse learn their native tongue. Others would be horrified that an outsider was being coached in their secrets. I don’t think it would be a monolithic disapproval though: Dwarves like an argument, they wouldn’t all uniformly agree.