Bug 14203: Message for non-existent lang removal

Bug 14203: Message for non-existent lang removal

A trivial string patch to update the error message displayed to
user if koha-translate is used to attempt removal of a language
that is not installed.

Test plan
=========

1/ attempt to remove a non-existent language by
   <installdir>/debian/scripts/koha-translate --remove <langcode>
2/ it should show "Error: the selected language is not already
   installed."
3/ apply patch
4/ repeat step 1; it should show "Error: the selected language is
   not installed."

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>