From GroupScript documentation
Which file is text '...' located in?
If you want to change wording somewhere (perhaps, you are located in U.S. and don't like a site with British spelling), you should do a search in system/application/views folder.
If you are using Windows, there is a wonderful and free utility called grepWin (http://tools.tortoisesvn.net/grepWin.html) which is far better than Windows built-in search tool. Please be aware that the files in system/application/views are HTML files with a bit of PHP mixed in - so you should have at least basic understanding about HTML and PHP (actually, you have to recognize PHP and not touch it if you don't know what you are doing)