We do not provide any language packages ourselves. Our software only ships with English language files. I assume that you are using a third party translation.
Assuming that the translation is built correctly – something we have no control over – the most likely problem you have is that the language package you are using is German for Germany (de-DE) while you are Austrian and are probably using the Austrian German (de-AT) language on your site's backend. The language code of your backend must match exactly the language code of the language pack for Joomla to use it.
Another issue might be that not all language strings may be translated. Any untranslated strings in your language will display in en-GB, the default language of Joomla and the one we are shipping with.
I'd recommend asking the people who published the language package for support. As I said, we don't publish language packages ourselves since two years ago and have no control over the content or the build process of third party language packs for our software.
Nicholas K. Dionysopoulos
Lead Developer and Director