The mother tongue of most of the people in the world is not English. It might be a widely used language like Chinese or French, or a rarely used language like the Bask language or Yiddish. These people, regardless of their English skills, may be potential clients; the only problem is how to reach them.
See how to Internationalizing Web applications using Dojo. This article presents a way to design, implement, and test the translation aspect of i18n.