As I added as note on issue 2113:

If it's possible to make the Extended Content not translatable, that would be even better! Than I could use a media picker together with Vorto for the translatable content. Switching languages in Merchello is not very user friendly currently, for example: why is it tucked away in the Actions button and only showing the active language in the footer/breadcrumb bar? If there's already plans for updating the interface (moving product info, extended content and product options to navigation tabs), please let me know!

Could someone point me into the right direction? Or is this currently not possible at all?

Our customer also has more than 10 website languages, but only adds product info in 3 languages. I really like the Vorto options and interface for adding these translations (adding only the required languages and having checkmarks when a translation is provided). Looking at the Merchello source, all languages are always returned: https://github.com/Merchello/Merchello/blob/31990060cd16517a3c4506c2a4458a5115a38737/src/Merchello.Web/Editors/DetachedContentApiController.cs#L233.