\documentclass[a4paper,12pt]{article}
\usepackage[LY1]{fontenc}
\usepackage[utf8x]{inputenc}
\usepackage{polski}
\usepackage[lf]{berenis}

\usepackage{listings}
\usepackage{xcolor}
\DeclareTextCompositeCommand{\k}{LY1}{a}
  {\oalign{a\crcr\noalign{\kern-.27ex}\hidewidth\char7}}
\begin{document}

\lstset{
    frame=tb, % draw a frame at the top and bottom of the code block
    tabsize=4, % tab space width
    showstringspaces=false, % don't mark spaces in strings
    numbers=left, % display line numbers on the left
    commentstyle=\color{green}, % comment color
    keywordstyle=\color{blue}, % keyword color
    stringstyle=\color{red}, % string color
    inputencoding=utf8x,
    extendedchars=\true,
    literate={ą}{{\k{a}}}1
             {Ą}{{\k{A}}}1
             {ę}{{\k{e}}}1
             {Ę}{{\k{E}}}1
             {ó}{{\'o}}1
             {Ó}{{\'O}}1
             {ś}{{\'s}}1
             {Ś}{{\'S}}1
             {ł}{{\l{}}}1
             {Ł}{{\L{}}}1
             {ż}{{\.z}}1
             {Ż}{{\.Z}}1
             {ź}{{\'z}}1
             {Ź}{{\'Z}}1
             {ć}{{\'c}}1
             {Ć}{{\'C}}1
             {ń}{{\'n}}1
             {Ń}{{\'N}}1
}

\begin{lstlisting}[language=C++,frame=single]
while(again == 1)
     //again er defineret som 1, loekken koeres igen.
    //początki bywają trudne
    cout << "Velkommen til BMI beregneren!" << endl;
    cout << "\nEr du en mand eller kvinde?
    Tast 1 for mand, tast 2 for kvinde" << endl;
    cin >> sex;
    cout << "Indtast din hojde i cm." << endl;
    cin >> height;
    cout << "Indtast din vaegt i KG." << endl;
    cin >> weight;
\end{lstlisting}

\end{document}