unless you are sitting in a corporate environment where Multi-Language
capabilities are licensed, I think the answer is: you cannot!
Normal editions for any Office version come in exactly one interface
language and proofing tools for a small selection of other languages.
If you need the latter for languages that are not included in your
edition, it depends on your version of Office whether you may buy the
proofing tools for _all_ available languages in one set/CD (up to Office
2003) or buy them separately (Office 2007).