I've also done this with the custom english version just so I could override a few strings. It works and is easier than hook_form_alter
, but it also creates a bit of unnecessary overhead, especially if your site is otherwise not localized/multilingual.
There is nowadays a light and easy solution, that uses the same locale system, but you don't need to setup a custom english language or code anything: the "String Overrides"-module.
http://drupalmodules.com/module/string-overrides
Surprisingly, it's one of the top rated modules, out of over 5000 module.