This article only covers how to do localization with a Core library of type PCL, I however, am using the Shared project type as the Core library, and it does not support .resx files, only .resw files. I've tried adding the resx file manually but I can't get the code generation working.
https://developer.xamarin.com/guides/cross-platform/xamarin-forms/localization/
One solution I am gessing is to create a new Class Library project and reference it, (PCL or normal since I want to use the Localization texts in a web project as well).