TranslateExtension.cs

[ContentProperty("Text")]
public class TranslateExtension : IMarkupExtension<bindingbase>
{
    public string Text { get; set; }
    object IMarkupExtension.ProvideValue(IServiceProvider serviceProvider, ILocalizer localizer)
    {
        return ProvideValue(serviceProvider, localizer);
    }

    public BindingBase ProvideValue(IServiceProvider serviceProvider, ILocalizer localizer)
    {
        var binding = new Binding
        {
            Mode = BindingMode.OneWay,
            Path = localizer.GetTranslation("<language here>")
            Source = ResourceManagerHelper.Instance,
        };
        return binding;
    }
}