It's great for language learning! I find they're useful for 'wide but shallow' reading comprehension. I'm so familiar with the stories I can pretty much guess what different words in the sentence mean, so I don't have to look up every other word in a dictionary. They provide a good stepping stone to materials aimed at native speakers, which are better for 'narrow and deep' language learning, where you might really have to do a lot of looking things up in dictionaries or grammar books. I started 'collecting' them when I was in high school learning French, and they had such a huge impact on my reading comprehension that even now, several years later, reading things written in French isn't a huge deal (though I'm not at a place I once was).

Obviously the best way to get foreign books would be to travel to the country where the language is spoken and pop into a bookshop, but for various reasons (particularly nowadays) that isn't always practical.

If you live in or near a major city, especially one with a diverse population, larger bookshops might have a foreign-language section, and you're bound to find at least a couple of the books in the series in a few different languages. If you can't find the one you want the shop might be able to order it in. A large bookshop near where I live includes both a Harry Potter section and a foreign-language section, so there's a delightful overlap.

But since booskhops are closed right now your best bet is online. If the language you're learning is a major, international one you can try the Amazon page for that country (France, Germany, Italy etc. all have their own versions) or for those countries without Amazon (South Korea or China, for instance) you might have to be a bit more creative and look for proprietary bookshop websites and see if they'll ship to where you are. It can be expensive as hell (in addition to the books themselves; my Japanese Harry Potter set cost me about $200) but if, like me, you love the series and the 'physicality' of the books themselves, it's a good investment.