Question marks in output string

I'm trying to replace some chars in my string but somehow I'm getting questions marks instead of chars.

StreamReader sr = new StreamReader(@"xxx.txt");
            string userInput = sr.ReadToEnd();
            string[] s = userInput.Split(' ', ',', '.', '\r', '\n');
            for (int i = 0; i < s.Length; i++)
            {
                if(s[i].Contains('ä'))
                {
                    s[i] = s[i].Replace('ä', 'a');
                }
                if (s[i].Contains('ü'))
                {
                    s[i] = s[i].Replace('ü', 'u');
                }
                if (s[i].Contains('ö'))
                {
                    s[i] = s[i].Replace('ö', 'o');
                }
            }

            foreach (var item in s)
            {
                Console.Write(item + " ");
            }
            Console.ReadKey();

What I'm expecting as output is something like this :a a a a a. What I'm actually getting: ? ? ? ? ?