Yes the first step is to choose the language to get plugin modules for (as you have done). Doing that should cause a new Update Center to be added, which will then get the two plugin modules for the language, which you need to install.
So try going to the top menu, Tools, Plugins, then the Settings tab. You should see an Update Center entry: "ThinkingRock German (Germany) Update Center".
Then go to the Installed tab and click on the Reload button. The language plugin modules should appear under the "Available" tab. Instal them. You may need to restart TR again (as the normal restart after installing modules does not re-read the configuration file).
I know it is a bit convoluted but we didn't want to cause everyone to download every language module and updates.