Manifest for WebExt dictionaries

Hi.

I have noticed that all dictionaries on AMO have been converted to
WebExtensions. I have updated our build scripts but I also got feedback
that the new format does not work well when there are two dictionaries
installed for the same locale.

Does anyone know, if the property keys of the "dictionaries" property in
manifest.json have to be valid ISO locale codes, or it can be anything?
See
https://github.com/MozillaCZ/cesky_slovnik_pro_kontrolu_pravopisu/pull/8.

Cheers,

-- 
Michal