The Multilingual User Interface (MUI) allows users to change the language of the user interface (UI). To make this possible, the MUI uses a single core binary that includes the system default language, together with one resource dynamic-link library (DLL) for each additional target language. The target device boots with the system default language and then a new user-selected language goes into effect after a soft reset. This switch requires recreating windows, menus, and dialog boxes with the newly loaded resources. In addition, to be considered successful, the language switch must display these elements with the correct fonts and with the correct locale-specific information.
Multilingual means a application which support different cultural languages. MUI means different user interfaces/Views for multiple languages
http://msdn.microsoft.com/en-us/library/aa913592.aspx
http://en.wikipedia.org/wiki/Multilingual_User_Interface