Fix for issue #4986. Translation broken in admin.

Issue detail: https://github.com/woothemes/woocommerce/issues/4986

This happens because there are a lot of strings of admin that resides in the frontend .mo file. And changes made in https://github.com/woothemes/woocommerce/commit/bb8fd9b50d0740569665466bb1fcf949138063a0#diff-355207be481b8c53c1d92f0c86ab1086 removes a line that loads the frontend .mo file at both sides, admin and frontend (if you have it at WP_LANG_DIR).

Changed loading preference too, in frontend loading, (custom) translation stored at WP_LANG_DIR must be loaded first (as it does in admin).