The first step is to choose the language to get plugin modules for under Preferences or Options/Miscellaneous/Language Tab. The drop down list contains only the language available currently.
Once a language has been selected, a new Update Center is added by the application. From that center, you will then get the two plugin modules for the language, which you need to install.
Go to the top menu, Tools, Plugins, then the Settings tab. For example, 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. Install them. You may need to restart TR again (as the normal restart after installing modules does not re-read the configuration file).
It is a bit convoluted but we didn't want to cause everyone to download every language module and updates.