How can I get true if we compare a to á?

You can use string.Compare() setting CultureInfo.InvariantCulture and CompareOptions.IgnoreNonSpace as you can see below I have created a method called CompareStrings(string str1, string str2), it will return a boolean

public bool CompareStrings(string str1, string str2)
{
    return string.Compare(str1, str2, CultureInfo.InvariantCulture, CompareOptions.IgnoreNonSpace) == 0; 
}

Calling the method to compare strings:

Console.WriteLine(CompareStrings("a", "á"));
Console.WriteLine(CompareStrings("a", "a"));
Console.WriteLine(CompareStrings("a", "b"));

Results:

True
True
False

The CompareOptions.IgnoreNonSpace definition: It "indicates that the string comparison must ignore nonspacing combining characters, such as diacritics. The Unicode Standard defines combining characters as characters that are combined with base characters to produce a new character. Nonspacing combining characters do not occupy a spacing position by themselves when rendered."

You can find out more about CompareOptions on docs

After normalization in forms D and KD, the string will contain two characters: a letter and a diacritical character. It is necessary to make a comparison with the letter.

string stra = "á";

string strFormC = stra.Normalize(NormalizationForm.FormC);
string strFormD = stra.Normalize(NormalizationForm.FormD);
string strFormKC = stra.Normalize(NormalizationForm.FormKC);
string strFormKD = stra.Normalize(NormalizationForm.FormKD);

Console.WriteLine("C {0}", strFormC.Length); // 1
Console.WriteLine("D {0}", strFormD.Length); // 2
Console.WriteLine("KC {0}", strFormKC.Length); // 1
Console.WriteLine("KD {0}", strFormKD.Length); // 2

Console.WriteLine("a".Equals(strFormD[0].ToString())); // True
Console.WriteLine("a".Equals(strFormKD[0].ToString())); // True

---

We can remove all diacritical characters with a regular expression.

\p{M} - is Unicode category means All diacritic marks.

string stra = "á";

string strFormD = stra.Normalize(NormalizationForm.FormD);

var result = Regex.Replace(strFormD, @"\p{M}", string.Empty);

Console.WriteLine("a".Equals(result)); // True
Console.WriteLine("a" == result); // True