TranslateExtension.cs

using System;
using System.Globalization;
using System.Reflection;
using System.Resources;
using Xamarin.Forms.Xaml;
using Xamarin.Forms;

namespace MyApp.il8n
{
    // You exclude the 'Extension' suffix when using in Xaml markup
    [ContentProperty ("Text")]
    public class TranslateExtension : IMarkupExtension
    {
        readonly CultureInfo ci;
        const string ResourceId = "MyApp.Resources.Resources";

        private static readonly Lazy<ResourceManager> ResMgr = new Lazy<ResourceManager>(()=> new ResourceManager(ResourceId, typeof(TranslateExtension).GetTypeInfo().Assembly));

        public TranslateExtension()
        {
            if (Device.RuntimePlatform == Device.iOS || Device.RuntimePlatform == Device.Android)
            {
                ci = DependencyService.Get<ILocalize>().GetCurrentCultureInfo();
            }
        }

        public string Text { get; set; }

        public object ProvideValue (IServiceProvider serviceProvider)
        {
            if (Text == null)
                return "";

            var translation = ResMgr.Value.GetString(Text, ci);

            if (translation == null)
            {
                #if DEBUG
                throw new ArgumentException(
                    String.Format("Key '{0}' was not found in resources '{1}' for culture '{2}'.", Text, ResourceId, ci.Name),
                    "Text");
                #else
                translation = Text; // returns the key, which GETS DISPLAYED TO THE USER
                #endif
            }
            return translation;
        }
    }
}