A few minutes ago, when I viewed profile pages on my phone, the tabs were all in Portuguese:
On the mobile interface:
On the full site
But it didn't happen on my desktop, and now it's back to normal. Other users report that the site is normal, but Seth says it's still happening, so I've decided to post this bug report even though it seems to be working for me at the moment. Perhaps it's some sort of intermittent problem.
I'm posting this on Meta.SE, since it seemed to happen on both Japanese.SE and ELL.SE, so it doesn't seem to be specific to a particular site.