Like others said, there's not an option to change the language in the Japanese 3DS, and Happy Home Designer doesn't seem to include more than one, so you'll be stuck with that language forever. As if I can't stress it enough, it's a very tricky language to learn in the eyes of an average English person due of its intricacies that aren't normally found in pretty much any Latin language, especially the kanjis when you don't have a good-enough learning method of them. Here, commitment is a must if you really wish to be able to fluently master the language.
Language barrier isn't the only thing that's potentially going to dampen the enjoyment of playing a Japanese-only game, but there's also the cost of importing the system, games and whatnot outside of your region, especially on anything related to current-gen video games. Japan normally price them a bit higher than in Europe, and you'll most likely have to pay fees on getting your import orders shipped all the way to your home, depending on where you live and the method of shipping. A Japanese standard 3DS and a typical import 3DS game compatible with the system's region will run you for at least $175 USD without the shipping fees, the latter of which may go up by at least $20 or even more. I don't have an accurate idea on the exact pricing due of the complexity of the customs that depends entirely on which country you're from and where you bought the product. One thing that's for certain, is that you'll end up paying more than you would if you just buy the same game in Europe for less.
Are you really importing everything you'll need just for one game? If that's the case, then I don't see that your import would worth much. Consider getting a few noteworthy 3DS games that are exclusive to Japan as well, such as E.X. Troopers, Taiko no Tatsujin games, Daigasso Band Brothers P, Puyo Puyo Tetris, any of the Dragon Quest Monsters remake, Time Travelers, Beyond the Labyrinth, Retro Game Challenge 3, Fire Emblem if (basically Fates in Japan), Inazuma Eleven 1・2・3!!, and more.
For further information on importing anything related to 3DS,
check this NeoGAF thread. Do some research before taking a decision.
Sorry if I somehow got off-topic, but that much of information is worth mentioning to further help you enjoying your import.