How to disable gettext feature?

You already have the answer, but perhaps you’re not running your code early enough.

Returning true from the override_load_textdomain filter will prevent all MO files from loading, but you’ll have to run it early to catch WordPress Core translations. Adding it as a must-use plugin does the trick for me:

/*
 Plugin Name: Disable Gettext
*/
add_filter('override_load_textdomain','__return_true');