example.xaml

<!-- Using static strings (like localized string) -->

<Label Text="{namespace:StringCase Input={x:static LocalizedStrings.MyString} StringCase=Upper}" />

<!-- Using binding -->

<Label Text={"Binding MyString, Converter={namespace:StringCaseConverter StringCase=Upper}" />

StringCase.cs

namespace Visit.Mobile.MarkupExtensions
{
    /// <summary>
    /// A string case extension that can be used in XAML with static values (like localized strings).
    /// Using this is the same as using a string case converter for bindings.
    /// </summary>
    [ContentProperty(nameof(Input))]
    public class StringCaseExtension : IMarkupExtension
    {
        public string Input { get; set; }
        public StringCase StringCase { get; set; }

        public object ProvideValue(IServiceProvider serviceProvider)
        {
            if (string.IsNullOrEmpty(Input)) return string.Empty;
            switch (StringCase)
            {
                case StringCase.None:
                    return Input;
                case StringCase.Upper:
                    return CultureInfo.CurrentCulture.TextInfo.ToUpper(Input);
                case StringCase.Lower:
                    return CultureInfo.CurrentCulture.TextInfo.ToLower(Input);
                case StringCase.Title:
                    return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(Input);
                default:
                    throw new ArgumentOutOfRangeException();
            }
        }
    }

    public enum StringCase
    {
        None = 0,
        Upper,
        Lower,
        Title,
    }
}

StringCaseConverter.cs

namespace Visit.Mobile.Converters
{
    public class StringCaseConverter : IMarkupExtension, IValueConverter
    {
        public StringCase StringCase { get; set; }
        public object ProvideValue(IServiceProvider serviceProvider) => this;

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (!(value is string stringValue)) throw new Exception("Input has to be of type string");
            if (stringValue == string.Empty) return string.Empty;

            var stringCaseExtension = new StringCaseExtension() { Input = stringValue, StringCase = StringCase};
#nullable disable
            return stringCaseExtension.ProvideValue(null);
#nullable restore
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}