Titles are broken in the Japanese RSS feed
as well, despite the markup itself having working Japanese text in it.
Was the database converted or migrated at some point? The page itself is encoded as UTF-8, and differently-encoded text in the page itself would present as mojibake
and not just a series of "?" characters, so it seems more likely that the database encoding got screwed up somehow and clobbered all the Japanese text. But without any more site privileges than a regular user, I have no way to tell if that's actually the case or not.