tesseract: Reintroduce enableLanguages

I've removed that attribute in 68bc260c

,
because the language files no longer were distributed as seperate files,
but if we for example only want to use the English training data, the
closure size of Tesseract gets quite large (around 1.2 GB), which is a
bit much just to be able to run NixOS VM tests.

For this reason I've also switched the VM tests back to using only the
English language.

Tested using the following VM tests (the ones that have OCR enabled) on
x86_64-linux:

 * nixos/tests/chromium.nix -A stable
 * nixos/tests/emacs-daemon.nix
 * nixos/tests/installer.nix -A luksroot
 * nixos/tests/lightdm.nix
 * nixos/tests/plasma5.nix
 * nixos/tests/sddm.nix

Signed-off-by: aszlig <aszlig@redmoonstudios.org>