Add command `export langfiles`

This adds a command to export language files to a given directory.

The purpose is to create a "backup repository" for language files that are handled on Transifex.

I'm not sure about the structure, so I replicated the one we have in the source code but I could imagine a more "flat" structure, like on Transifex.
The current structure would make it possible to just "copy and paste" the language files over the source code.

Of course this could also be tweaked to export the languages separately and zip the files up - aka "creating language packs" :wink: :wink: