Any language version of Windows XP or Windows 2000 can be selected as a platform for the test if the product is properly globalized. Of course, in the case of localization testing, the localized version of the operating system can be a wise choice, since that's the most likely environment for your application in the real world. However, a globalized and localizable application, even after it undergoes localization, must be able to run on any language version of the operating system and with MUI installed.
You should run the application with MUI installed when your application implements an MUI behavior, through pluggable UI, satellite dynamic-link libraries (DLLs), or some other technique that adjusts the UI language to the user's preferences. MUI allows the user to switch the UI language of the operating system and thus you must make sure your application matches the operating-system settings. You should verify the behavior of the application when the user's default language of the UI differs from the other locale settings. By doing so, you'll immediately see any problems in the way resources are loaded and processed.