framework/filters/ContentNegotiator.php
protected function isLanguageSupported($requested, $supported)
    {
        $supported = str_replace('_', '-', strtolower($supported));
        $requested = str_replace('_', '-', strtolower($requested));
        return strpos($requested . '-', $supported . '-') === 0;
    }
protected function isLanguageSupported($requested, $supported)
    {
        $supported = str_replace('_', '-', strtolower($supported));
        $requested = str_replace('_', '-', strtolower($requested));
        return strpos($requested . '-', $supported . '-') === 0;
    }
Returns a value indicating whether the requested language matches the supported language.
@param string $requested the requested language code
@param string $supported the supported language code
@return bool whether the requested language is supported