C++ easy question Classes

[arturs]

I have a MainWindow class and in the class I defined in public part QString app_language

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    QString app_language;

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    QString app_language;

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I would like to access to QString app_language; from another Class

I tried to use the following code:
MainWindow.app_language;

but It does not work

How to do it ?

Regards
Artur