You can see what locales you have on your system by using the following command:
locale -a
If you can't find the preferred setting then you must generate it, which is very distribution dependent. To do so, a package like glibc-i18ndata (openSUSE, Mandriva, ...) or locales (Debian, Ubuntu, ...) must be installed then enter the below command as root to generate the language_COUNTRY.UTF-8 locale. For example the Brazilian is:
localedef -i pt_BR -f UTF-8 pt_BR.UTF-8
Then you can use the "locale=pt_BR.UTF-8" option during mount.