I've never used Office Language Packs, always only the English Office with multilingual Office Proofing Tools, which gives spelling, thesaurus and grammar for additional languages (I need Finnish, Swedish, German and English if not daily, at least weekly).
The terminology used is a bit misleading. MUI
(Multilingual User Interface), LIP
(Language Interface Pack), LP
(or CLP, Complete Language Pack), all are systems used to allow another dialog and interface language than installed original. Here's a very good page to read if you are really interested: Language Packs in Windows 7
. It's not just about LP's in Windows 7, it also helps you to understand the difference between various multilingual technologies.
MUI technology was presented in Windows 2000, but it was quite simple until Vista. Today, XP does not have complete language packs, they are only offered as free, optional updates to Vista and Seven, Ultimate and Enterprise editions.
As far as I know, you can have MUI interface packs in all Office versions since Office XP.