Windows Vista offers two options to change the User Interface (UI) language:
- MUI (Multi Language Interface) Packs
- LIP (Language Interface Pack)
The MUI packs offer the possibility to introduce multiple languages in a single Vista installation and choose a language per user. Users can change UI language whenever they like to. MUIs are only available for Windows Vista Enterprise and Ultimate editions. The MUIs for Vista Ultimate are available for download from Windows Update. The MUIs for Vista Enterprise must be obtained from the Volume Licensing Program.
LIPs provide the possibility to change the UI for Home Premium and Business versions of Windows Vista. A LIP changes most parts of the UI and is probably less complete than the MUI. A LIP changes the UI for the complete OS and offers no possibility to change languages after installation of the LIP.
LIPs seem to be one of the hardest add-ons to find on the Microsoft site. The Microsoft Local Language Program website seems to offer LIP downloads, but does not get any further then Windows XP and Office 2003.
A full list of available LIPs is located here. The only problem with the list is that it is not complete and download locations are not provided.
Fortunately an MCT colleague (thanks Johan) located quite a number of available LIPS that I will be listing here:
- Chinese (PRC)
- Chinese (Hong Kong SAR)
- Danish
- Dutch
- English
- Finnish
- French
- German
- Italian
- Japanese
- Korean
- Norwegian
- Portuguese (Brazil)
- Russian
- Spanish
- Swedish
You can see that there Bulgarian is still missing 
.