Apple is very considerate in providing language localizations for many, many languages. But the chances are you aren't using the Catalan, Farsi, or Klingon language localizations on your Mac. And even if you are, you can strip out the other ones you aren't using and gain more efficiency and drive space using a utility like Monolingual.

By the way, the latest version of Monolingual will also strip out unnecessary architecture from your system - but not your applications - so it also performs a function similar to XSlimmer, yet different.