In the absence of the "userlang" query parameter in the URL, the value
of the "Accept-Language" header is used. However, it is assumed that
"Accept-Language" specifies a single language (rather than a comma
separated list of languages possibly weighted with quality values).
Example:
Accept-Language: fr
// should work
Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5
// The requested language will be considered to be
// "fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5".
// The i18n code will fail to find resources for such a language
// and will use the default "en" instead.
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