public class TranslationExtension : IMarkupExtension<string>
{
    public static ResourceManager Manager { get; set; }

    public string Key { get; set; }

    public string ProvideValue(IServiceProvider serviceProvider)
    {
        if (Manager == null)
            throw new InvalidOperationException("ResourceManager property 'Manager' has to be set");

        if (string.IsNullOrEmpty(Key))
            return "";

        return Manager.GetString(Key) ?? $"[{Key}]";
    }

    object IMarkupExtension.ProvideValue(IServiceProvider serviceProvider)
    {
        return ProvideValue(serviceProvider);
    }
}