Bug 721449 - Remove language black magic in OS X build

The language on OS X is properly set in GIMP also without the black
magic code. To improve maintainability we remove it and also improve
comments.

(cherry picked from commit 556e9ac83e6208c00ae69e5302efe440dc63d86b)
Bug 721482 - Add missing language files

Add the missing language files for GEGL and GIMP to the gtk-mac-bundler
configuration.

(cherry picked from commit fb743b5a095d1523f281011a600a2ff2465182e9)