Could you please update Polylang to the latest version?
That's a known bug of 1.2 & 1.2.1
http://wordpress.org/plugins/polylang/changelog/
You will have to delete your cookie for the bug correction to be efficient.
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Chouby on "[Plugin: Polylang] browser language according is not working, $_COOKIE[pll_language] not working"
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