I just wrote a brief blog about this. Feel free to have a look. Suggestions are welcome.http://nnhiu.blogspot.com/2014/07/resx-xamarin-forms.html
Thanks @NguyenNhuHieu, your post and video make it really simple to understand.
For those who have issues with Xamarin Studio on Mac, it's because of the default configuration of the .Net naming policies, which doesn't follow the one of Visual Studio, so the resources cannot be loaded.
You can find more information here: http://forums.xamarin.com/discussion/comment/78851/#Comment_78851
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