Multilingual User Interface (commonly referred to as MUI) is a Microsoft technology for internationalization and localization. As of September 2010 an MUI implementation is not planned for ReactOS, though Edison Andreassy has expressed interest in developing MUI for ReactOS.
Stefan Fulea is working on an alternative - see CORE-10335
The MUI technology is covered by an international patent titled "Multilingual User Interface for an Operating System".
This patent is most likely expired, but this needs to be verified.