It sounds to me as if you have one website and you want each ‘site’ to have the same content but just in its own language at its own URL.
You can achieve this via one core installation and a multilanguage plugin (I’ve used QTranslate with some success).
Unless you want to maintain separate sets of structures, you don’t need to turn on the multisite features (or have more than one installation).
Bear in mind that you might need to do some clever redirection if you want there to be more than one path to a language (e.g. /us and /uk would both be English).