[#577] Set formencode's translation languages

Tell `formencode` to use CKAN's `ckan.locale_order` config setting instead
of getting one for itself from the system.

This fixes a couple of tests in `ckan.tests.lib.test_navl` that would fail
if your computer was not set to English.

Changing formencode's translation languages in one module seems to change it everywhere we import formencode. I just did it in the main module where we seem to be using formencode, just to keep all the formencode usage in one place. I'm not sure if this was the best place to do this