I'd like to say that I feel uncomfortable with the quality of translation as well.
What the end user sees is the frontend and the wild mixture of languages. I know from my experience that this makes it much more difficult to argue pro Plone as the impression he gets is the one of poor quality even if you have the most sophisticated techstack.
I opened a topic a few days ago and the screenshot there is taken from Plone 5.08 - so this problem exists for several years now. The answer I got was to do some coding.
I am not quite sure if the need to be a developer is adequate just to edit some strings in the frontend.
I would do some translations english->german to get a better coverage if this is helpful, but I won't take a course in coding for this task.
Finally I'd say that as being multilingual is a promoted feature of Plone this problem is a marketing desaster.
Plone is great and has a wonderful community but it lacks the focus on the customer (the enduser) in my opinion.