Almost all applications (which use gettext, at least) are written with their C (untranslated) locale in en_US. This means that they never have an explicit en_US translation, but are always 100% translated to en_US.

Should the <languages> element list en_US explicitly, in which case it should probably be injected by the appstream XML generator?

Or should the spec say that the application’s C locale is never listed as a <lang> element, and should be assumed to be en_US?

If we go with the second option, perhaps the assumption about the C locale being en_US could be overrideable (in the few cases where an application is written in something other than US English by default) by setting an attribute like <languages default-language="es_ES"> and defining that language as 100% translated.

I can put together a merge request to update the specification either way, but need to know what approach would be preferable 🙂