Just put the one for your site to German, and the one for admin to English, and make sure there is nothing ruining it inside override/site.ini.append.php.
You'll want to put ContentObjectLocale to the same on both though, judging from your post I'd say German.
ContentObjectLocale dictates what language should be the default for objects you create.
Thanks very much! I'm just beginning to understand the INI settings. I also found a good tutorial for multilanguage sites that made me understand. For all those looking for answers I recommend:
