The easiest way to contribute to TuxGuitar project is to write a translation file for a new language. If your language is not supported you are free to make a translation. You can check out supported languages on http://tuxguitar.svn.sourceforge.net/viewvc/tuxguitar/trunk/TuxGuitar/share/lang/. Please, take translation process with care.
You have to know: TuxGuitar is a living open-source project. New features will require new translations, so making a translation file will require further activity from you!
It is easy to test your translation: simply put your messages_LANG.properties file in $TUXGUITAR_DIR\share\lang folder and it will appear in languages next time you run the application.
When your translation file is done and ready for use upload it somewhere and send a link to the author via TuxGuitar contact page.
Besides translating the main TuxGuitar application, there is also an option for translation of the plugins. Very few plugins are translated so far, but you are free to do it.