I'd be tempted to think that there is *some* preference lurking somewhere in a non-discoverable location, or some bugged cookie or something like that... but if they really are using some kind of inference model instead of explicit preferences, that's messed up.
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For me, I find that if I need info only available in Japanese, I'll be searching for it in Japanese anyway, and that returns Japanese results. Not having a language in that list is not a hard filter. On the contrary, having it seems to give undue weight to that language.
I get the feeling that Google search heavily skews towards other languages as soon as you tell it you're interested in them. Which probably makes sense, because lots of people wind up on English by default. But it doesn't help those of us who have no interest in localized content