The plist editor in Xcode seems to insist that it should be a string ... if you want to get an array, try opening the plist file in a text editor and adding it after the value:
<key>CFBundleLocalizations</key> <array> <string>English</string> <string>French</string> </array>
so your plist will look like this:
... <key>CFBundleExecutable</key> <string>${EXECUTABLE_NAME}</string> <key>CFBundleLocalizations</key> <array> <string>English</string> <string>French</string> </array> ...
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