in the meantime i was able to make some kind of workaround. i was curious and looked deeper into the gwenview problem when i saw, that exiftool and exiv2 are doing there job exactly the same.
i overlooked that also exiftool shows the "ü" wrong in the tag:
"Exif.Photo.UserComment Undefined 46 Deutschland, Speyer, Kleines Fl�sschen" (this is the output from exiv2)
and both tools show the comment how it should in the tag:
"Xmp.dc.description LangAlt 1 lang="x-default" Deutschland, Speyer, Kleines Flüsschen"
so i did again a look into the php code. it works in dokuwiki when i use everytime the xmp.dc:description tag. for this i had to extend the "array('IPTC.Caption','EXIF.UserComment',..." thing used in the gallery plugin and the mediamanager to the list below. at every place only the 'Simple.Title' tag was read i replaced it also with the full array list.
here is what was at line 1090 in the inc/template.php file:
i replaced it with:
this works for me an my workflow using digikam which writes the comment to exif, iptc and xmp tagging systems. it also needs a modification in the template (details.php), thats why i can only declare this as a workaround.