+1 on keeping .properties. Switching to another format should be a goal by itself, with equals thoughts provided on the developer side AND the localizer side. You should weight such move by proving that it introduces a significant improvement for both point of views.
If you are interested in localization, I encourage you to take a look at all what has been done by kaze from gaia team for firefox os localization. He came up with .properties files on steroïd.