It's impossible to use the undo action (Ctrl-Z, etc.) after the application of a link in ContentTranslation.
There's no description, so I'm not sure what does it mean.
If it means that a link in the translation cannot be converted to plain text, that it's wrong - there's a "Remove link" button in the sidebar when you click a link.
Sorry, I mean automatic undo as in CTRL+Z. As you only have native undo from CE you will encounter its bugs, such as not being able to undo past a programmatic change to the DOM. VE has its own transactions and undo stack so we avoid this problem.
This is already supported in CX2, the major refactoring and architectural update of Content Translation to integrate the same editing surface as VisualEditor. Currently it can be tested by adding &version=2 to the URL of the translation editor (still experimental), and will become the default version in the future.