Like below? Note that really you should take the culture from the arg
to ConvertTo / ConvertFrom - but I was in a hurry ;-p

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;
using System.ComponentModel;

class MyDisplayNameAttribute : DisplayNameAttribute
{
   public MyDisplayNameAttribute(string displayName) :
base(displayName) { }
   public override string DisplayName {
       get { // TODO: localize the display name
           return "Cultured " + base.DisplayName;
       }
   }
}
class MyBooleanConverter : BooleanConverter
{
   private readonly string trueVal, falseVal;
   public MyBooleanConverter()
   {
       trueVal = "vrai"; falseVal = "faux";
   }
   public override StandardValuesCollection
GetStandardValues(ITypeDescriptorContext context)
   {
       return new StandardValuesCollection(new string[] { trueVal,
falseVal });
   }
   public override object ConvertFrom(ITypeDescriptorContext context,
System.Globalization.CultureInfo culture, object value)
   {
       string sVal = value as string;
       if (sVal != null) {
           if (sVal == trueVal) return true;
           if (sVal == falseVal) return false;
       }
       return base.ConvertFrom(context, culture, value);
   }
   public override object ConvertTo(ITypeDescriptorContext context,
System.Globalization.CultureInfo culture, object value, Type
destinationType)
   {
       if (destinationType == typeof(string) && value != null &&
value is bool)
       {
           return ((bool)value) ? trueVal : falseVal;
       }
       return base.ConvertTo(context, culture, value,
destinationType);
   }
}
class Foo
{
   [MyDisplayName("Some Prop")]
   [TypeConverter(typeof(MyBooleanConverter))]
   public bool Bar { get; set; }
}

static class Program
{
   [STAThread]
   static void Main()
   {
       Application.EnableVisualStyles();
       Application.SetCompatibleTextRenderingDefault(false);
       Form form = new Form();
       PropertyGrid grid = new PropertyGrid();
       grid.SelectedObject = new Foo();
       grid.Dock = DockStyle.Fill;
       form.Controls.Add(grid);
       Application.Run(form);
   }
}