Pietro,
Go to your site-map /admin/site-map, edit the parameters for acs-lang and set UsePackageLevelLocalesP to 1. This should display the community in the locale which is defined in the apm_packages table, default_locale column.
I can't recall now if there was a UI to change a community's locale though or if I made a new one for a demo site we did that's specifically for showcasing different classes with localized content.