Problem/Motivation
install_begin_request() checks for the existance of $GLOBALS['config']['locale.settings']['translation.path'] to set up the correct string translation service.
This, however, is not the correct way to specify a global configuration override. I.e. even if you set that global, \Drupal::config('locale.settings')->get('translation.path') will return the default value.
Proposed resolution
Check for $GLOBALS['config']['locale.settings']['translation']['path'] instead.