How to support more language in gdm login?

[Godfrey]

Hi happy ubuntu users,

When one login in, you can click on Language, then whill be provided a
list of  languages from the "select a language for your session
to use" drop down menu.  But now I only have these "Last", "System
default", "English American" and "POSIX/ C English" on the list, which
means that I can only choose English.  My question is how do I set the
system up, such that it will include all the natural languages to
choose from, such as Spanish, Germany, Japanese, Chinese, etc.