How can I translate the field group description?
As far as I know there is no proper UI for this yet, but field groups are config, so you should be able to translate all strings by creating/editing translated versions of those config files.

First, export your current config, e.g. drush config:export

Then, in your config/sync directory look for files like core.entity_form_display.ENTITY.BUNDLE.FORMMODE.yml. In those files you will see a structure like

langcode: ISO2CODE
status: true
depenencies:
  config:
    - ...
third_party_settings:
  field_group:
    GROUP_ID:
      children:
        - field_foo
        - field_bar
      label: MY_GROUP_LABEL
      format_settings:
        description: 'MY DESCRIPTION'
...some more stuff...


The third_party_settings/field_group contains your group labels in your primary language. To translate a label, create a file language/LANGCODE/core.entity_form_display.ENTITY.BUNDLE.FORMMODE.yml below your config/sync directory.

This file must contain the same structure as the one in the main config file, but remove everything that is not translateable:

third_party_settings:
  field_group:
    GROUP_ID:
      label: MY_TRANSLATED_LABEL
      format_settings:
        description: 'MY TRANSLATED DESCRIPTION'


Finally, reimport the config with the translated labels, e.g. drush config:import