when calling lang::util::convert_to_i18n, do not always register a en_US translation, which will be otherwise always overridden. Instead, let lang::message::register make sure that a en_US message exists and create it only when needed as a fallback. Before, lang::message::register would return an error when e.g. a new message keys was registered for de_DE and no en_US was there. Now the en_US will be just created on the fly.