@Dylan Tomorrow - I'm certainly not opposed to supporting multiple languages on AO3. And you're correct that the first step towards that would be to get them to add it to the displayed story metadata.
To my eye, the best way to do that would be to add it to the metadata block in: '<dl role="complementary" class="work meta group">' But that's clearly their choice to make.
(FYI, AO3's list-by-language pages are broken--they all give the same 'recent updates' list. Click a few of the 'works' links on the languages
to see it. So there's no way to judge how accurately stories in different languages are tagged.)
As for default language, except for the few language specific sites and ffn, FFDL does default the language to English('en') inside the epub. It doesn't set the language in calibre in that case. Something about that feels familiar, like it's come up before--I'll check old postings and get back to you.