Sorry for the update, I just wanted to let you know that the jQuery Migrate Helper plugin workaround it's not working either... sometimes I get the text, sometimes not.
It seems to be a script order sequence issue.
It surely is a huge issue for my clients which are not developers and cannot write HTML.
As a temporary workaround, I managed to find this "procedure" that works:
1. once in the translation editor, set all editor's to the Text tab
2. do something (like translate the page title), click save
3. refresh the page (via browser), all editors should now have the Text tab as active tab
4. now, if you switch any editor to Visual, tinyMCE will load
As said, switching between Text / Visual is the editor's have the Visual tab active first, doesn't work.
Additionally, I have to say that the jQuery Migrate Helper plugin generates tons of unwanted/useless Ajax requests (like, hundreds of requests) on some pages, which cause the server to respond with 508 Resource Limit Reached error. So that's not a viable plugin to use.