Apache mod_rewrite cookie based localization

.htaccess

RewriteEngine On

# Set locale cookie if lang query string is set (90 days expiry time)
RewriteCond %{QUERY_STRING} lang=(.*)
RewriteRule .* ?locale=%1 [cookie=locale:%1:.%{HTTP_HOST}:129600:/]

# Append locale to query string if:
# - Query string hasn't already been rewritten
# - Locale cookie is set
# - Request URI isn't a valid file (for assets etc.)
RewriteCond %{QUERY_STRING} !locale
RewriteCond %{HTTP_COOKIE} locale=([^;]+)
RewriteCond %{REQUEST_URI} !-f
RewriteRule ^(.*)$ $1?locale=%1 [QSA]

index.php

echo $_GET['locale'];