I use GRASS 6.4.2 on Russian version of Windows 7 x64. As I start wxGUI it is shown in Russian. But I don't like its translation and want to work with English interface. I have no clues how to solve it.
We have recently added a language selector in the graphical interface. This is currently under testing in GRASS 6.4.3. For now, you can upgrade to GRASS 6.4.3svn but installing this version:
In the wxGUI:
Settings --> Preferences --> Appearance --> Language Settings
It should solve your problem.