For those who are curious, WESTRING stands for World Edit String per Nichilus' post. These are intended to change based on your game's Language setting, depending on your game version. Having "Missing String" error should not stop you from doing anything as those string files used by World Editor does not have entries for the requested keys. You might also encounter those errors if you are using higher patches (preferably 1.31 or Reforged), but your game locales doesn't match the ones used by your system/OS.
Extreme measure: Using either MPQ Editor or Casc Viewer, extract "UI/WorldEditStrings.txt", add missing strings, then place your modified file inside the game root (Local Files must be enabled first).
Last resort: Reinstall the game from the disc if you are using the older versions or use the "Scan & Repair" feature in Battle.net Launcher if you are using the current patch, which redownloads the corrupted files.
One of these should solve the problem for you.