You cannot inform them really. You might open an issue, but I haven't found a way to list all issues so that you can click on them either. Transifex assumes that if a string is reviewed that people won't really want to change it anymore, although a reviewer can still edit it or bump it back to unreviewed. So, if you wish to use a review process, I think this is the best way to go about it: only mark strings as reviewed when you have double-checked them carefully, and if you come across a reviewed string that you think needs further discussion, unreview it. Of course, translators without reviewer powers won't be able to unreview a string, so they will need to use a different means of catching reviewers' attention.
How about a thread in this forum called "German translation issues" where everybody posts strings to look at? You could also link this thread on the Wiki. For discussing a particular string, you can link to individual entries on Transifex - just get it in the editor and then copy it fro the browser's address bar, like this: https://www.transifex.com/projects/p/widelands/translate/#de/scenario_emp01/40698817?q=barbarian