Currently, MgnlUserManager always create users with the "en" language.
This should be configurable globally (i.e a defaultLanguage property in the config node of the UserManager (/server/security/userManagers/XXXX/defaultLanguage and the appropriate setDefaultLanguage() in the implementation)

Another approach might be to simply not set that property at all, and ensure we do fall back on the language defined by config:/server/i18n/system/fallbackLanguage in that case.