Language::isAvailable now also work if you pass a lang without charset.
This fixes the language field of configuration screen.

issue #6051

issue #6065

issue #6051

No more global_un_set, global_delete or delete

issue #6024

And convert global_get calls to get

issue #6024

Also global_is_set to is_set.

issue #6024

Used by webservice for application/json

issue #5989

issue #5955

issue #5956

Signed-off-by: Benoit Mortier <benoit.mortier@opensides.be>

issue #5854

issue #5831

Renamed all languages to include a country as well, to match debian
 locale names.
Also added a fallback on the 2-letter locale in all cases. (falling back
 still triggers a PHP error)

issue #5819

issue #5820

issue #5820

When no language at all was asked by the browser and language was set to
 automatic FD would select Arabic because of sorting. Now it will
 default to English

issue #5820

This also fix the workflow to make sure the language in the header is
 correct in all cases.
A PHP error is triggered if setlocale fails or if the wrong language
 header was sent.

issue #5820