Here is a piece of code that seems to do what you're looking for - if I understood it correctly :-)

    public static char? FromKeys(Keys keys, InputLanguage inputLanguage)
    {
        return FromKeys(keys, inputLanguage, true);
    }

    private static char? FromKeys(Keys keys, InputLanguage inputLanguage, bool firstChance)
    {
        if (inputLanguage == null)
        {
            inputLanguage = InputLanguage.CurrentInputLanguage;
        }

        byte[] keyStates = new byte[256];

        const byte keyPressed = 0x80;
        keyStates[(int)(keys & Keys.KeyCode)] = keyPressed;
        keyStates[(int)Keys.ShiftKey] = ((keys & Keys.Shift) == Keys.Shift) ? keyPressed : (byte)0;
        keyStates[(int)Keys.ControlKey] = ((keys & Keys.Control) == Keys.Control) ? keyPressed : (byte)0;
        keyStates[(int)Keys.Menu] = ((keys & Keys.Alt) == Keys.Alt) ? keyPressed : (byte)0;

        StringBuilder sb = new StringBuilder(10);
        int ret = ToUnicodeEx(keys, 0, keyStates, sb, sb.Capacity, 0, inputLanguage.Handle);
        if (ret == 1)
            return sb[0];

        if (ret == -1)
        {
            // dead letter
            if (firstChance)
            {
                FromKeys(keys, inputLanguage, false);
            }
            return null;
        }
        return null;
    }

    [DllImport("user32.dll", CharSet = CharSet.Unicode)]
    private static extern int ToUnicodeEx(Keys wVirtKey, uint wScanCode, byte[] lpKeyState, StringBuilder pwszBuff, int cchBuff, uint wFlags, IntPtr dwhkl);