How to add translation file to Qt in Ubuntu

I am not able to create a translation file in a Qt Creator Widgets project.

In main.cpp, mainwindow.cpp and mainwindow.h, I have:

#include <QTranslator>

In the pro file I have

TRANSLATIONS = lang_en.ts

I should note that:

• I cannot start Qt Creator from the program search but only from the terminal.
• I have also tried new empty projects.
• I can add one translation file when creating a new project.

When I execute lupdate from the GUI it prints:

Starting external tool "/usr/lib/qt6/bin/lupdate /home/moritz/C++/Georechner.pro"

When I execute:

lupdate -verbose ~/C++/GeoRechner/Georechner_aktuell/Georechner.pro

in the terminal, it prints:

WARNING: Could not find qmake spec 'default'.

And no .tr file is to be found. I suppose I am missing some Qt language installation or update or something of the kind?

I'm using Ubuntu 22.04.

Related:

• How to add translation files to Qt

• Qt 5 Documentation: Hello tr() Example

• How to add translation file to Qt in Ubuntu

Janjua's answer from here did it: lupdate doesn't create .ts translations file

On my system the lupdate file to execute (because there are several), is

/home/moritz/Qt/6.6.0/gcc_64/bin/lupdate
so in total :

/home/moritz/Qt/6.6.0/gcc_64/bin/lupdate ~/path/to/project/projectname.pro