I’m using https://addons.mozilla.org/en-US/firefox/addon/deepl/ in combination with Gutenberg editor. I mark one paragraph, use the addon shortcut, then can select and modify the translation in the addon window, just like on the deepl webpage. Then I copy the translation, close the addon window and paste the text.
This is quite quick, even though you must go through the whole document paragraph by paragraph.
Certainly it would be nice to automate this, or even integrate into the editor. But so far, I have found no plugin or solution compatible with WP Multilang.