Using Resource files in XAML in Xamarin.Forms

Creating RESX files within Shared projects is known to cause issues. You can see several lengthy posts in Xamarin Forums regarding this (here and here for example).

The easiest solution that will allow you to use the approach you want is to create a new .NET Standard Library of PCL library in your solution, create your RESX files there and set their visibility to public. Then you will be able to utilize them using the x:Static syntax as expected.

Many developers use an alternative in the form of a custom markup extension like the solution by ClaudioPereira in this forum. This simplifies the syntax even more.

Finally, for most detailed information on Xamarin.Forms you can refer to the official documentation.