Tracking correctly available translations (take 2)

Tracking correctly available translations (take 2)

The previous scheme of loading all possible translations and checking
whether the work is a bit too much "brute force" and causes problems
on Mac OS X (documents loaded with the wrong language).

Now there is an helper static method in Messages class that checks
whether a readable .mo file exist for the language. There should be an
API in gettext for doing that, but alas it is not possible.

As a consequence the method Language::translated() has been removed,
along with its cache.