SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-custom-resource-provider-considerations

CustomResourceProviderClass.cs

using System.Globalization;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web;

namespace SitefinityWebApp
{
    public class CustomResourceProviderClass : IResourceProvider
    {
         public object GetObject(string resourceKey, CultureInfo culture)
         {
             if (SystemManager.CurrentHttpContext == null)
                 return "Compile time value";
             
             //custom code
             return "some object";
         }

        public string GetResourceContent(string resourceName)
        {
            throw new System.NotImplementedException();
        }

        public bool TryGetResourceContent(string resourceName, out string resourceContent)
        {
            throw new System.NotImplementedException();
        }
    }
}