If you would rather not reinstall, I'd try first searching the registry and seeing if it is just a simple registry setting, if not you could in theory use MPQView and extract EVERYTHING from ur .mpq's that is in the /ENG/ folders. Save them to their appropriate paths, as they appear in MPQView and then add -direct -txt to your diablo 2 shortcut. Round about way of doing things but it should work
*Edit* I guess its possible that d2lang.dll in your diablo folder has something to do with it, If you can find the english version of d2lang.dll logic says copying this file to your d2 directory should switch the language... I'll have to get home where my install CD is before I could verify this.