I have two field sets
Field groups are configurations which are not translated via the translation interface. I am quite sure that the string "Account settings" that is translated in your project does not come from your field group. Just change the label of a field group to any other string - you will not find the string in the translation interface.
You need to activate the "Configuration Translation" module.
https://www.drupal.org/files/issues/2020-02-05/2546212-119.patch - Did the trick for me. It adds a "Translate form display" tab, like it is shown below.