Indeed, that's also a problem! We've discussed this with coders, and I'm sure we will be discussing it more in the future, as they're redesigning the way browsing and searching works in the archive. (In the meantime, I would advice anyone to use advanced search (http://archiveofourown.org/search?), which lets you use language as one of the parameters.)
I'm not sure one could say what's the biggest problem that non-English users could encounter at the archive, because it would depend on the community, but I know a lot of them wouldn't be able to use the archive at all without an interface in their language, even for posting. This of course probably varies wildly between languages and countries, depending on the level of English that is imposed in obligatory education (I know that for example Argentina's level is awful -- I doubt anyone from here could just navigate the archive with that alone. Even my mother, who is college educated and mostly handles herself, had issues with it when I made her try a couple of things).