int main()
{
string test = "Danish letters: Æ Ø Å !!!!!!??||~";
cout << "Test = " << test << endl;

for(int l = 0;l<test.size();l++)
{
    if(!isalpha(test.at(l)) && test.at(l) != ' ')
    {
        test.replace(l,1," nope");
    }
}

cout << "Test = " << test << endl;

return 0;

Test = Danish letters: Æ Ø Å !!!!!!??||~
Test = Danish letters nope  nope nope  nope nope  nope nope  nope nope nope nope nope nope nope nope nope nope nope"

test.at(l)!='Å'