silva-extra metadata set should be using language infrastructure for Zope 3

[Martijn Faassen]

Right now, the language codes are hardcoded in silva-extra. We can use the z3
locales infrastructure hopefully to display this better. The languages could
also be displayed in the language of the user's preference. This is already
being used by the language selection in the user settings screen, and the code
needs to be consolidated.

[Kit Blake (kitblake)]

See issue 1668 for the complete story, I'm adding this now so we include it
whenever this change gets made.

I changed the case of the abbreviations in the metadata set. And then I got an
error in the metadata screen.

Fortunately I could use the back button, set the language to none, (i.e. "Select
a Language") and then set it again. Thus, for the release documentation:

NOTE: If the language in your properties screen has been already set (probably
in your site root) you will get an error in that screen, after importing the
metadata set.
Error Type: KeyError
Error Value: 'Wrong item value [[EN]]'
You MUST first change the setting to none (i.e. "Select a Language") and save.
Then when you import the metadata set and reload the properties screen you can
set the language appropriately.

[Eric Casteleijn (thisfred)]

the hardcoded codes are lowercased in 1.5 and 1.6, changing to the zope 3
locales will have to wait a little.

[Sylvain Viollon (thefunny)]

This is a limitation of Formulator. When metadata will be refactored not to use Formulator, this limitation will be gone.