You then need to work on each such property file and translate messages. You don't have to translate the entire file — if you leave some entries empty, they'll fall back to the default locale.
Quick locale switcher firefox extension is useful to toggle between various locales.
Once you made some changes, you can commit them. Translators should consider themselves as owning property files for their locale, so feel free to go ahead and just commit. If you are new to this, doing a small commit first is a good idea. You can also always send in a patch if you prefer to be safe.