Detect primary language by HTTP Accept-Language.

detect-primary-language.php

<?php
$accepted_languages = explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE']);
$primary_language = $accepted_languages[0];
if ($pos=strpos($primary_language, ';') !== false) {
    $primary_language = substr($primary_language, 0, $pos+1);  // Throw away quality value.
}
if ($pos=strpos($primary_language, '-') !== false) {
    $primary_language = substr($primary_language, 0, $pos+1);  // Throw away dialect.
}

header("Location: http://mysite.com/$primary_language"));