For a list of currently enabled locales, run
$ locale -a
To enable a certain locale, un-comment or add the relevant lines in
/etc/default/libc-locales and reconfigure
Some programs have their translations in a separate package that must be installed in order to use them. You can search for the desired language (e.g. "german" or "portuguese") in the package repositories and install the packages relevant to the applications you use.