Even the order of entries in the Accept-Language header, as per browser settings? That's usually the source of half the problems I have with websites suddenly defaulting to an unexpected language.

(The other half is websites suddenly displaying in Norwegian; it started after I was in Oslo for a few days and, while there, had to re-activate Skype from the website. Even a decade and two jobs later, on completely unrelated computers, some websites will suddenly display in Norwegian, for reasons completely unknown.)

No need to worry then, it's merely a bug :)