Based on T144167#5075490 it seems some users copy content from Content translation to paste it into Visual Editor. As a result unnecessary attributes leak into the final result. Based on this example you can see that unnecessary HTML markup was removed such as the following:
<span data-segmentid="9" class="cx-segment">...<span>
This task is intended to:
- Explore if there is a way for Content translation to reliably clean up contents when they are copied.
- Check that any clean-up approach does not cause issues when pasting the contents in Content translation itself because of the lost metadata. This would limit the ability users have to move content around.
- Check that the solution works when both using the copy&paste clipboard and drag&drop.
- If there is no reliable solution from Content translation side, explore how to clean-up the contents when pasted into Visual Editor. Similar approaches may be in place for pasting content from other tools such as Microsoft Office.
Users may be doing this as a shortcut to expand existing articles with a translation of some new content, but that's just a guess. We don't know how often this behaviour is.