Trust me, this type of question keeps popping up time and again in this forum, so I would guess you haven't done your part seriously, however, as you ask, though I'm not an expert either, if you are strictly talking about n900 and not the future releases then C/gtk+ would be your best bet as the C is the main language it depends on. There are however, other languages too like python etc, but I'm not sure about "java" yet, you can start development with and I'm sure C/gtk+ would be helpful in extra work as well. Anyway, if you are thinking about future releases as well then Qt will be what maemo will be based on and it is cross-platform too.
EDIT: Well!! there you go..