Errors 81, 82 or 84 displays: "The client does not match the accounts native language.Please, use the game native client." There is no known fix as of yet, as Blizzard controls the language restrictions from their servers.D2Multi Res installation is notoriously finicky, especially when trying to use it alongside other tools.The second of the linked guides is a video tutorial specifically aimed at joining "The Sin War" (Median's online realm) that while good is probably a bit too long and the form of video doesn't lend itself to quick skimming of information.
The third installment in the Diablo franchise, Diablo III, is a dark horror/fantasy-themed role-playing game (RPG) developed and published by Blizzard Entertainment.
Apparently, 64-bit users will not be able to play the game, as there are some 32-bit Open GL libraries missing.
As a side note, make sure you meet the minimum system requirements (especially the graphics card and CPU/RAM) for playing Diablo III before going through the tutorial!
If you're new and trying to install Median (and maybe play on realm) you may have stumbled upon two other guides in this forum: this and this and you may be wondering what guide should you follow, and why.
The first of the linked guides was born to help players to install an utility called D2Multi Res, which lets you play Diablo II in resolutions different from the standard 640x480 or 800x600.