Given the content of an 'Accept-Language' request header, returns an array of the languages.

This differs slightly from other language functions, as it doesn't try to validate the language codes,
or merge similar language codes.

@param  {string} header - The content of the AcceptedLanguages header.
@return {Array} An array of language codes in the header, all in lowercase.
function getAcceptedLanguagesFromHeader( header ) {
	if ( ! header ) {
		return [];
	}

	return header
		.split( ',' )
		.map( lang => {
			const match = lang.match( /^[A-Z]{2,3}(-[A-Z]{2,3})?/i );
			if ( ! match ) {
				return false;
			}

			return match[ 0 ].toLowerCase();
		} )
		.filter( lang => lang );
}