Ooo, this is an interesting topic!
I don’t think my family totally understands it, but they’re pretty supportive anyway My parents have supported my weird hobbies/obsessions since childhood, so it’s just another weird hobby to support, I guess. They buy me the Japanese learning books that I ask for as Christmas/birthday presents, and my mom’s loaning me some money to go on a trip to Japan in a couple months. No interest in learning it themselves, as far as I can tell, but at least I can talk to them about it and they won’t shut me down. My immediate family’s pretty open-minded and intellectually curious about random topics, so they don’t mind.
Other than that, I pretty much don’t tell people that I’m learning Japanese. In my experience, it goes one of two ways: 1) they expect you to be instantly amazing at it, and they get disappointed when they find out you’re not, or 2) they don’t get it, and you get that subtle, disdainful "…ok, weirdo " look.
I feel like it is kind of a weird hobby for me to have because I don’t have a “socially acceptable reason” to be learning it (I don’t have any Japanese friends/family members, I don’t need it for my job, etc.). But why does there have to be some big justification for it? Do I ask you why you’re so into Crossfit, which honestly sounds like a new form of torture to me? Do I ask you why you enjoy going bar-hopping, which is something I 100% do not see the appeal of? No, I don’t. It’s a hobby. It’s fun for me, and Japan is interesting. Get over it. But people don’t, so I say my hobby is reading or Netflix and move on.
I have a friend who probably would have learned it with me if I had started while we were still in school together (she was kind of the one who put the idea into my head), but we’re in this kind of weird, limbo place in our friendship right now because we don’t live that close anymore and we’re both awkward people, so…it’s a long story, but I haven’t really told her I’m doing it. I’m in a weird place with friends in general right now, because I’ve been changing locations, work has been taking up a lot of my time, and I honestly don’t know how to make new friends as an adult. The only people I regularly talk to are people at work, which…see above.