Actually it's pretty good development practice to start an SaaS app with localization from day one. Why wouldn't you? The world doesn't revolve around English speakers (there are more non-English speakers than there are English speakers, at least what their first language is concerned), the same as it doesn't revolve around people with 100% vision.

You start an editor with extensibility from day one, as well as theming from day one, as well as multi-platform from day one, etc.

Why should it be different for Localization or Accessibility?