Partially english texts after update

I upgraded an older installation to 2.9.2 via the wordpress autoupdate function. The upgrade itself worked fine, but after that my german installation shows some texts in english instead of german in the frontend.

For example: “by [author] on” is not translated, but “edit” is translated correctly to “bearbeiten”.

I already searched the web for a solution and came up with some possible problems, but none of them seemed to be the cause:

* I upgraded to the german language version
* I checked correct upper/lowercase of the language setting in wp-config and the names of the po/mo files
* the theme I use (hybrid and child) has a translate feature via po/mo files, those are translated
* php and mySQL are above version 5.
* I found a problem where a 64bit Linux server could cause this with Pytal, but that was for an older version of wordpress. the solution was to change core files which I am reluctant to do. The file “wp-includes/gettext.php” ist not there in my installation anyway. Could this be the problem or does the file no longer exist in 2.9.x?

Any help please? Thanks in advance.

Xanathon