Default Language: pt_PT.mo x pt_BR.mo

Hi,
I'm using a MP MU 3.05 intall (made by psek.com), and I need to change language setting to Brazilian Portuguese (pt_BR.mo). Portuguese ( pt_PT.mo) is similar, but not the same. Usually, that's a simple task, but in my WP MU install it shows only "Portuguese" in the Default Language, even if I copy a pt_BR.mo to /wp-content/language/ directory (wich I downloaded from Wordpress.org inside the wordpress-3.0.5-pt_BR.zip).
Thanks in advance.