You are probably looking for an answer for Xcode 4.X, but I hope this helps.
In Xcode 3.2.6, in the "Groups and Files" section of the project window, open the "Executable file" section. Select the executable file that you want to run in English, and execute the command I. Click the "Arguments" tab, and then click "+" at the bottom. Then add:
-AppleLanguages ββ"English"
as an argument.
Hth
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