PHP: Get Browser Language, optionally passing a list of available languages.

get-browser-language-code.php

<?php
/**
 * Get browser language, optionally passing a list of available languages.
 * 
 * @param  [array]   $available_languages  Available languages for the site
 * @param  [string]  $default              Default language for the site
 * @return [string]                        Language code
 */
function get_browser_language_code( $available_languages = [], $default = 'en' ) : string
{
	$browser_languages = explode( ',', $_SERVER['HTTP_ACCEPT_LANGUAGE'] );

	if ( empty( $available_languages ) ) {
		return substr( $browser_languages[ 0 ], 0, 2 );
	}

	foreach ( $browser_languages as $language ) {
		$language = substr( $language, 0, 2 );
		if ( in_array( $language, $available_languages ) ) {
			return $language;
		}
	}
	return $default;
}

Suggestion for line 15: return substr( $langs[0], 0, 2 );
And I would use !empty() instead of isset().