What is a mui file and how do I open a mui file?

MUI, or Multilingual User Interface files are resource files used by Windows operating systems versions 2000 and later. MUI files contain resources that allow Windows interface to display different languages. While files with the MUI extension are mostly associated with the Windows Multilingual User Interface, they can also be created by an Open GL toolkit used to write OpenGL programs. The Windows Operating System can open .MUI files, dependent on the instruction provided to the O.S. by the user. Windows will choose the specific .MUI to use based on criteria and selection. MUI architecture separates the language resources for the GUI, from the binary code of the operating system. The separation makes it possible for the user to change languages completely without changing the core binaries of operating system, or to have more than one languages installed on the same computer while utilizing the same core binaries. Languages are applied as language packs containing the resources required to localize part of or the entire user interface.
The level of support available for a multilingual user experience at the Windows os level and for multilingual application development on the Windows platform evolved over time and across the different iterations of Microsoft Windows.
Prior to the release of Microsoft's Windows Vista, the supported functionality was fairly basic, with single-language Windows images and the option to add-on Multilingual User Interface Packs in specific conditions. There was no developer support for any multilingual applications whatsoever.  With the release of Windows Vista, Microsoft made a significant investment in .MUI, and Windows Vista was built from the ground up on a .MUI platform. While this represents a significant advance in Windows localization strategy, as it was a key enabler for Microsoft to provide Windows in more languages than ever before, it was first and foremost a great advance for Windows users, developers, and customers.