Please use http://translatewiki.net for localisation. Your base file is extremely dirty and contains a lot of messages still in English. It is completely unsuitable to merge with any current version of the german messages file. I'll take a look at the plugins later; those are not yet supported by translatewiki.net

MantisBT localisation statistics: http://translatewiki.net/wiki/Translating:Mantis/stats/trunk

I didn't know that mantis uses translatewiki now. But I don't understand why my base file is "extremely dirty". As you can see by a look into my script and into the original (english) file, I updated the old strings_german.txt so it has the same structure (order) as the english file. And I transferred all previous work in the old german file to the new one before adding missing.

Anyway: translatewiki version seems to be more up-to-date -- maybe "mantis unstable" could be updated on mantisbt download page (next milestone). Or is there a script where I can update my language files?

You can grab http://translatewiki.net/w/i.php?title=Special%3ATranslate&task=export-to-file&group=out-mantis&language=de which will give you the most recent version for German.

With "dirty" I mean the following and more showing up in your new file version (does not look like German to me):
$MANTIS_ERROR[ERROR_GENERIC] = 'An error occurred during this action. You may wish to report this error to your local administrator.';
$MANTIS_ERROR[ERROR_SQL] = 'SQL error detected. Please report this to ';
$MANTIS_ERROR[ERROR_REPORT] = 'There was an error in your report.';
$MANTIS_ERROR[ERROR_NO_FILE_SPECIFIED] = 'No file specified.';
$MANTIS_ERROR[ERROR_FILE_DISALLOWED] = 'The file type is disallowed.';
$MANTIS_ERROR[ERROR_NO_DIRECTORY] = 'The directory does not exist. Please check the project settings.';
$MANTIS_ERROR[ERROR_DUPLICATE_FILE] = 'This is a duplicate file. Please delete the file first.';
$MANTIS_ERROR[ERROR_DUPLICATE_PROJECT] = 'A project with that name already exists.';
$MANTIS_ERROR[ERROR_EMPTY_FIELD] = 'A necessary field "%1$s" was empty. Please recheck your inputs.';
$MANTIS_ERROR[ERROR_PROTECTED_ACCOUNT] = 'This account is protected. You are not allowed to access this until the account protection is lifted.';
$MANTIS_ERROR[ERROR_ACCESS_DENIED] = 'Access Denied.';

Yes, I did not translate the error messages, just $s_*

Anyway, I will grab the new file and merge it into my installation. Thanks for the hint.

Added the translations for the plugins to git master; removed the untranslated/same strings. Ignored the core messages submission, as stated in earlier comment.