.NET (Core and Framework)

using System;
using System.Globalization;

namespace AccentInsensitive
{
  /// <summary>
  /// Summary description for Class1.
  /// </summary>
  class Class1
  {
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main(string[] args)
    {
      CultureInfo ci = CultureInfo.InvariantCulture;

      string s1 = "aA eE iI oO uU";
      string s2 = "áÁ éÉ íÍ óÓ úÚ";

      for (int i = 0; i<s1.Length; i++)
      {
        string cmp1 = new string(s1[i],1);
        string cmp2 = new string(s2[i],1);
        int res = ci.CompareInfo.Compare(cmp1, cmp2, 
                                         CompareOptions.IgnoreNonSpace | 
                                         CompareOptions.IgnoreCase);

        if (res < 0)
          Console.WriteLine("{0} < {1}", cmp1, cmp2);
        else if (res > 0)
          Console.WriteLine("{0} > {1}", cmp1, cmp2);
        else
          Console.WriteLine("{0} = {1}", cmp1, cmp2);
      }

      Console.ReadLine();
    }
  }
}

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