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.
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
description: 'MY DESCRIPTION'
...some more stuff...
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
This file must contain the same structure as the one in the main config file, but remove everything that is not translateable:
description: 'MY TRANSLATED DESCRIPTION'
Finally, reimport the config with the translated labels, e.g.