Dictionary<char, string> EnRus = new Dictionary<char, string>
{
{'A', "А"},
{'B', "В"},
{'C', "С"},
{'E', "Е"},
{'H', "Н"},
{'K', "К"},
{'M', "М"},
{'O', "О"},
{'P', "Р"},
{'T', "Т"},
{'X', "Х"},
{'a', "а"},
{'c', "с"},
{'e', "е"},
{'o', "о"},
{'p', "р"},
{'x', "х"},
};
string text = "AАBВ";
foreach(char t in text)
{
if(EnRus.ContainsKey(t))
text = text.Replace(t.ToString(), EnRus[t]);
}
return text;