Configure Languages not working, The default language "English" must be translateable

Hi again - just try to get little bit further with Tryton Docker
After installation you run trought the Modul Configuration process, first window is “Configure Languages”.
But If you choose a language you will get error:
Configure Languages not working, The default language “English” must be translateable

I don’t understand how to handle this? Would be nice to get more informations about this behavior. Thanks

Tryton Docker Image 7.2

by the way: what exactly is the “load translations” and “unload translations” function in the desktop-client? It this maybe set and clean? Translation — Tryton server

If you have setup the database with “English” as default language (which is the default), you must keep it as translatable.

It toggle the translatable box and load or unload corresponding translation.