First and foremost, do you have a device (N8x0 for Maemo4/Diablo or N900 for Maemo5/Fremantle)? Translation without one is not fun and may produce low quality results.

I've been talking a bit and up so far the conclusion seems to be that it's better to wait a few weeks, since a better community en_US localization is being worked on at the moment, and I guess will help provide translators with a few hints about where to start and such.

I don't know much about the topic myself, but I suggest you start by familiarizing with GNU gettext terminology at least (.mo files, msgunfmt, etc.) since most of Maemo uses it for i18n purposes.