The WE expects comments to be stored with the local language setting.
iTunes, on the other hand, only accepts comments with the English denotification.
So if you use the WE as music database then you have to modify the language file that you use for MP3tag (can be found in the folder lang - if you are not sure where to find it, you can start a file search and look German.lng - open the corresponding folder and look for "your" language file.)
In this file - which is a plain text file, so open it with a text editor - search for
modify it so that the "eng" is replaced with your language identifier, e.g.
Check if the comment now show up properly when you save the comments again/anew.
A personal note: THe Windows Explorer is no suitable tool for tagging or maintaining an audio collection.