From Site5Wiki
One of your two webmail choices as a Site5 customer is Squirrelmail; the other, Horde.
[edit] Customization
[edit] Choosing a display language
Squirrelmail's localization preferences
As shown, from within Squirrelmail, click the
Options choice at the top in order to reach this screen, then simply choose from the pulldown menu (highlighted in green), and save with your other preferences available on this screen.