In recent updates Divi started using the frontend builder everywhere.
Even the backend internally loads the frontend builder in an iframe.
The consequence is that all the labels in the User Interface are displayed in the language of the post we are editing, instead of the administration language. This stems from the fact that WPML frontend language is determined by the post you are viewing. And the language that you select in User Profile only works for backend pages.
This only affects manual translation and its not a problem when the translation editor is used.
As a workaround, navigate to Divi → Theme Options. Go to the Builder tab then open the Advanced tab. There, disable the “Enable The Latest Divi Builder Experience” option.