Hi Greg,
Fantastic!
The process begins with the Translation Utility.
The utility will present you with every file that ExpressionEngine has available for translation. Follow the instructions in the User Guide.
One key step that is missing in the User Guide: when you are finished with your translation, you must group all of the translated files into a folder named after the language. In your case, polish. Once all the translated files are in the polish folder, move the polish folder to /system/expressionengine/language/ and then you should have Polish available as a language in the Control Panel.
Cheers!