If you should encounter the problem that some translations are wrongly translated with the equal source language string (for example in our case there were German translations for "Author" translated with "Author" or "Published" with "Published"), you may use the following snippet to list them.
SELECT s.lid,s.source, t.translation FROM `locales_source` s
INNER JOIN locales_target t
WHERE s.lid=t.lid AND CONVERT(s.source USING utf8) = CONVERT(t.translation USING utf8)
AND t.customized=1
To finally delete them, you may use something like this, but make a backup before and know what you're doing!
DELETE FROM locales_target WHERE lid IN (SELECT lid FROM (SELECT s.lid FROM locales_source s
INNER JOIN locales_target t
WHERE s.lid=t.lid AND CONVERT(s.source USING utf8) = CONVERT(t.translation USING utf8)
AND t.customized=1) temp);
Afterwards you may perhaps want to update your translations the following way: http://www.vaerenbergh.com/blog/drupal-8-reimport-all-translation-files
It's working - thanks!