Well, the rule is that you have to generate the .po translation file of the theme and place the file in ./oc-content/themes/{theme_name}/languages/{language_code}/theme.mo (it's a compiled file generated with poedit). This is the reason why bcute is showing "Select a category". It's not very different of how Drupal, Wordpress and other software works.
About the translation, if you want to help to correct the german translation, tell me and I'll create a new account to you in admin.osclass.org