Is it better to create a class instance, or create an instance each time a method is called?
Use an Attribute

The class should be tagged with:

[ValueConversion(typeof(Boolean), typeof(String))]


If it's of the normal, old-school .NET variety (desktop, client profile, WP8). Windows store apps don't appear to have an analogous attribute in their .NET libraries, and thus this recommendation can be ignored when compiling such a program.

Drop Type Checks

If the passed-in value is not of the expected type, you should not do your conversion; you should allow an exception to be thrown, so that you know an invalid binding has taken place (and you can go correct the offending code). This applies to both conversion and back-conversion.

Explicit Back Conversion

Check for an explicit match -- don't default to returning "false" on a non-match with the CALENDAR string. Instead, check that the input string matches with the FISCAL string. Throw an exception if the input string matches neither acceptable string.

Use a Singleton

The documentation for Windows.ApplicationModel.Resources.ResourceLoader implies that the default constructor you're using should result in an object that is identical in function to the one available from the static GetForCurrentView() function. If not, then set up your own singleton pattern to ensure that you have at most one shared copy of the resource object around at any given time.