Software should be in your native language, that's why translations for Pigeon Planner are open and everyone can help. Visit the "Translate" link in the links-menu that will bring you to the Launchpad translation page. You must first be registered to Launchpad and you are good to go.
If you don't want to register, download the latest translation template here or download a specific language file here if it already exists, so you can edit or add strings. A nice way to edit these files is with a program like Poedit.
Pigeon Planner is written in Python and uses GTK+ as its interface toolkit. Everyone is welcome to submit patches for every part of the program. The code is completely available through the Launchpad Bazaar branch, which can be found on the download page.