I am preparing to develop a web application that will (hopefully) be used by an audience with many different native languages.  
What should I do to prepare my software project to have the user interface be almost entirely internationalized?  
Are there any software stacks that make this easier?
One big change in your project: you need to build in time to localize your strings.  Depending on your schedules, you may have to rearrange work so that all the string-changing work is at the front so that you have time to get the translations done before launching.
Or, alternately, you have to build in a buffer at the end to account for it.
Preparing a web site for international usage