You can set default locale, time zone, and character set for the site.
Locale represents a set of user interface properties -- including language and time zone, for example -- that are often related to the user's geographic region. The locale setting determines what language UI text is displayed in. It also determines how dates are formatted, what the character encoding is, and so on. The correct locale helps to make people's experience in the community feel more familiar and comfortable.
Admin Console: System > Management > Locale
When you change the locale on the Locale Settings page, you're setting what is actually the default, or last fallback, among the contexts in which the locale can be set. This is a global default setting that can be overridden at three other levels that are narrower in scope. This global setting is only used when locale isn't set in the other places.