Chrome sets it if you tell it you want to use its spellchecker with some languages.
I'm not sure what the standard says, but i suspect you'd surprise users considerably less if you used the first language listed in that header rather than the last (assuming equal or missing q values).
I'm not sure what the standard says, but i suspect you'd surprise users considerably less if you used the first language listed in that header rather than the last (assuming equal or missing q values).