Support x-default as a language code.

[Berdir]

Right now, if you have exported default content with a specific langcode like en, and then install with a different langcode, your entities will exist but all their field values will be lost as they're stored in a translation for a non-existing language, or something.

This can be avoided by setting the language to und.

I'm wondering if it would make sense to support an explicit x-default in the exported files, not sure what's the best way to deal with it, the easiest might be string placements on "x-default", or maybe a larger pattern before unserializing it, but also hacky.