Xcode localization frustrations - Blog - Use Your Loaf

Xcode does a lot to make it easy to localize project resources such as strings and NIB files. There is one small annoyance that is worth being aware of though if you want to maintain your sanity. Since Max OS X 10.4 the preferred way to identify a language is with the ISO language designations. To be precise the two letter codes as defined by ISO 639-1 should be used where possible. So English is identified by en, Spanish by es, French by fr, etc.