In German there is a formal ("Sie") and an informal ("Du") form. I would like to translate some software into informal German, notably woocommerce using transiflex, but new languages can only be added using their locale code.
There only seems to be de_DE as a locale. What's best to differentiate between the two forms, shouldn't there be another locale code just for the informal form, too?
Generally Gettext uses @ to distinguish language variants. So in this case it could be de_DE@informal. This way the locale will be correctly handled by Gettext with fallback to de_DE (in contrast with suggested de_DE-x-informal which will not fall back this way as Gettext will see DE-x-informal as country code).
You can find more on locales naming at https://www.gnu.org/software/gettext/manual/html_node/Locale-Names.html