TranslateExtension for Xamarin.Forms Xaml. Note: the xmlns declaration in the <ContentPage> tag

fr.lproj\Localizable.strings

/* FRENCH translation*/

/* NOTE: because i'm using Vernacular, the 'Vernacular_P0_' prefix is not required in the Xaml expression. */

/* Untranslated: <new task> */
"Vernacular_P0_60newtask62" = "nouveau";

/* Untranslated: ApplicationHeading */
"Vernacular_P0_ApplicationHeading" = "LionTodo";

/* Untranslated: Delete */
"Vernacular_P0_Delete" = "Supprimer";

/* Untranslated: Done */
"Vernacular_P0_Done" = "Terminer";

/* Untranslated: Name */
"Vernacular_P0_Name" = "Nom";

/* Untranslated: Notes */
"Vernacular_P0_Notes" = "Notes";

/* Untranslated: Save */
"Vernacular_P0_Save" = "Enregistrer";

TodoItem.xaml

<?xml version="1.0" encoding="UTF-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:i18n="clr-namespace:QuickTodo;assembly=QuickTodo"
    x:Class="TodoXaml.TodoItemXaml">
    <ContentPage.Content>

    	<StackLayout VerticalOptions="StartAndExpand">

	        <Label Text="{i18n:Translate Name}" />
	        <Entry Text="" x:Name="nameEntry" Placeholder="task name" />

			<Label Text="{i18n:Translate Notes}" />
	        <Entry Text="{Binding Path=Notes}" x:Name="notesEntry" />

	        <Label Text="{i18n:Translate Done}" />
	        <Switch IsToggled="{Binding Path=Done}" x:Name="DoneSwitch" />

	        <Button Text="{i18n:Translate Save}" Clicked="OnSaveActivated" />
			
			<Button Text="{i18n:Translate Delete}" Clicked="OnDeleteActivated" />

	        <Button Text="{i18n:Translate Cancel}" Clicked="OnCancelActivated" />

	        <Button Text="{i18n:Translate Speak}" Clicked="OnSpeakActivated" />

        </StackLayout>
    </ContentPage.Content>
</ContentPage>	

TranslateExtension.cs

using System;
using Xamarin.Forms.Xaml;
using Xamarin.Forms;
using Vernacular; // I'm using Vernacular to translate, you might have your own custom class
 
namespace QuickTodo
{
	// You exclude the 'Extension' suffix when using in Xaml markup
	[ContentProperty ("Text")]
	public class TranslateExtension : IMarkupExtension
	{
		public string Text { get; set; }
	
		public object ProvideValue (IServiceProvider serviceProvider)
		{
			if (Text == null)
				return null;

			// Do your translation lookup here, using whatever method you require
			var translated = Catalog.GetString (Text); 

			return translated;
		}
	}
}