I have a project in Xamarin which targets Android, iOS and windows phone. I used core (PCL library) to share common code between different platform. I added Resource files (.net
For localization on our Xamarin projects I've used the Multilingual App Toolkit (detailed here) from Microsoft and T4 templates to transform the output from the toolkit to useable formats for Android and iOS.
This tutorial has a fantastic overview of the process and it's based on this code project.