Localize the installer

This makes it possible for the admin to choose their preferred language in the first step of the installer. That choice applies to the rest of the installation process and is set as the default system language in the config file.

Notes:
 * only the languages with a translated Install.php file will be presented for choosing
 * the locale for the default admin account is still being set to en-US for some reason
 * this doesn't update the templates used in the installer, which aren't fully localizable at the moment.

See merge request !47