File Extension MUI
File extension MUI used by Windows OS to store Multilingual User Interface. MUI files are internal windows files and are only used by Windows software. Typical MUI file will contain definitions and layouts of specific user interface elements which depend on region or language of the Windows operating system in use.
MUI could also mean Micro User Interface library which is part of OpenGL. Typically OpenGL MUI library doesn't produces files with MUI file extension thus file with MUI extension will typically be a Multilingual User Interface.
Frequently MUI file extension could be appended to an end of the already existing file. For example Myfile.dll could become Myfile.dll.mui which will store language specific definitions for Myfile.dll. One language specific file is used per supported language.