Yes, you can!

There is a pretty good article on the wiki about remapping the hw keyboard: http://wiki.maemo.org/Remapping_keyboard

For the virtual keyboard, there is an app called ukeyboard in the repos.

I, for one, created a full Hungarian hw keyboard mapping. This allows me to write öüóőúéáűí on the N900's hardware keys pretty easily.
(I remapped the Fn + arrow key combinations and the currency symbols to achieve that.)