I have my system configured in a language that most of my peers don't speak. The problem is that iCal then sends out all "invitation accepted" emails in that language as well. So I wanted to change the language, but just for iCal.
This can be achieved by opening the application's property file in ~/Library -> Preferences. For iCal, it's the file named com.apple.iCal.plist. Once the file is open, select the root node and add a new child with the name AppleLanguages of the class Array. To that node, you append a child node with the name 0, type String and your language as the value.
To see what the languages are called, you could open the file .GlobalPreferences.plist in the same directory, which contains the list configured in the System Preferences. (The names are two-letter abbreviations, with en for English, de for German, es for Spanish, etc.)
[robg adds: I haven't tested this one...]
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