Ubbi Dubbi translator in c++

I am starting learning how to code now and I saw this website

http://ift.tt/1zEbOKO

which is Ubbi Dubbi translator and I am making my own now to however, I am stuck on the crucial part where I need to add ub after vowel sounds

here is my code ( just some functions that I tried)

bool contains_only_letters(string str)
{
    for (int i = 0;  i < str.length()  ; i++)

        if ( (str.at(i) <'a' || str.at(i) >'z') && (str.at(i) <'A' || str.at(i) >'Z'))
            return false;


    return true;

}
bool is_vowel(char ch)
{
    return (ch =='A' || ch =='E' || ch =='I'|| ch =='0'|| ch =='U'
            || ch =='a' || ch =='e' || ch =='i'|| ch =='o'|| ch =='u');

}
string translate_to_Ubbi_Dubb(string str)
{
    string translated;

    if (contains_only_letters(str))
    {

        if (is_vowel(str.at(0)))
         translated = str + "ub"; *// here is where I don't know how to add ub*

    }
    else
        translated = str;

    return translated;

please help I wanna get a hand of it. By the way, this is c++ Thanks ^^