[translations] reason for translations not showing up / contributing

Hello, I have a Drupal installation with English and German translations. For some reason, some email texts, which are translated and parts on the website (I'm using a wide range of modules) don't show up when changing the language settings to German. Is this a known bug or does anyone know what could be a reason for such behavior?

Also, our translator started adding translations to the de.po exported from our installation. I've looked around on the Drupal website, but couldn't find info how to filter/export newly translated strings for individual modules out of our general de.po file to be able to upload it into the cvs. I'd be thankful for any pointers. regards,

Tomi