Adding a new language to 770/N800 is very simple. Once done, you can choose it from the "Device language" list of the "Language & region" applet.
1. install extra locales, got from http://garage.maemo.org/projects/locales-extras
2. add the country code to gconf configuration database, e.g.
# gconftool-2 --set /apps/osso/applet/languageregional/available_languages \
--type=list --list-type=string "[zh_CN]"
an alternative way to do this is using gconf-editor.
of course, the text of UI need to be translated to this new language.