After updating ACF plugin to version 5.8.5 or newer, it is no longer possible to save a translated version of Field Group. If you click the “Update” button on the field group edit screen nothing happens. The issue exists only on the translated field groups edit screen.
You can downgrade ACF plugin to 5.8.4.
Another solution, if you are familiar with code editing, is that you can fix the issue by editing the file wp-content/plugins/advanced-custom-fields-pro/includes/admin/admin-field-group.php in your WordPress installation. In this file, you find the function post_submitbox_misc_actions() and at about line 413 you need to change:
The same change will come with ACF updated to version 5.8.8.