There's a big difference between doing social media voluntarily for a fan group/project and doing social media as a job. It is not as simple as localizing posts from the Japanese accounts since even there you are not machine translating like fansites often do, but rather making sure the content matches with how the games are localized for your region.
I am very aware of this, but regardless, it should not be a problem for a company like Nintendo. After all, the Japanese branch of the company has no problem doing it.
Also, while I know it’s certainly not the same as a professional social media job, I ran my group (which wasn’t just some tiny unknown fan group, you know—it was one of the largest StreetPass groups in the country with thousands of followers and hundreds of active members, which is a lot for a relatively niche local meet up group—and in addition to that I also ran the accounts for a social media campaign that went viral internationally and actually prompted a response from Nintendo, if the StreetPass group isn’t enough) and its social media accounts as if it was one, adhering to the same kind of standards that you’d see on Nintendo’s official accounts in a way that, frankly, most fan accounts simply don’t—ensuring posts are positive and professional, using proper official terminology and correct spelling, punctuation, and grammar, and, yes, ensuring that posts about games are consistent with the actual games themselves (as someone who actually plays all Nintendo games that part comes easy), in addition to other expected duties like running ads and managing insights, creating images and videos, coordinating events, etc. And while we didn’t really deal with reposting Japanese content in English, if we had I wouldn’t have ever just thrown something into a machine translation and regurgitated it on our accounts, and it’s kind of insulting to assume I would have (though not an unreasonable assumption, I suppose, because plenty of fan accounts do just that).
Anyway, my point is I’m not totally detached from the reality of what’s expected from running an official social media account, and the fact is that managing additional accounts for the series that have them in Japan is
absolutely something that should be very well within Nintendo of America’s ability. And localization from Japanese to English is kind of
their thing, so I can’t imagine why you’d expect localizing Japanese posts to be a problem for them.