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).
In the new scheme, autotools install a file lib/installed_translations that contains a list of installed languages (the .gmo files that got installed). This file is read
in Languages::readInstalledTranslations and allows to set the translated() property
of each language.
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