For a list of currently enabled locales, run

$ locale -a

Enabling locales

To enable a certain locale, un-comment or add the relevant lines in /etc/default/libc-locales and reconfigure the glibc-locales package.

Setting the system locale

Set LANG=xxxx in /etc/locale.conf.

Application locale

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.