Language changes while editing order in backend

Hi everyone,

I've searched through the whole forum but couldn't find a post to my problem.

I've installed OpenCart with English and German. Everything is working well in the frontend. If orders are done in German they are saved in German and the customer's getting a German mail, if they are done in English, they are saved in English and the customer's getting an English mail. The backend is completely in German, thats how I setted it. So far so good.

If I change a german order in the backend, especially something in the ordered products, it changes immediately to English.
Not the backend is english, but any information about the changed order like, product names, shipping- and payment-information are shown in English.
How does it decide to change to English and is there a way to influence or manually change this decision, e.g. with an extension.

My opencart-version is 1.5.6
I had some qMods installed but deactivated them so far.

I'm looking forward to your help.
Regards