I've looked into this, and it seems the Translation API currently cannot access the settings_data.json file. Since this is currently where content is stored when a merchant populates a field for a sections setting, the Translation API can't be used here. What you could do is add more content to the existing theme locale files and reference these values to translation keys in the code of your theme.
For the new sections architecture, content for section settings will not be stored on the theme level, but on a store level, as a page resource. These fields and resources are currently not accessible by the Translation API, but we are looking into the best way to make these translatable and multi-lingual.
I hope this helps,
@Liam thank you for the explanation.
It makes sense and answers my question.
Playing with Translation API I was unable to access and translate the current sections content.
We are all really looking forward for the new sections architecture :)