can anyone of you give me a recommendation on how to learn programming robotic arms like the ABB IRB 120?
Based on the rest of your post I will recommend you take a official abb course, at least a basic one to familiariaze with the robot
Is there a special book about the used language, an interesting film or a good source on how to to use IRC5?
There are many videos and literature on the web. Just search for them
Or perhaps even a program simulating the coding process of a real robot?
Abb offers Robotstudio
Although I'm quite firm in other programming languages I haven't programmed a robot yet and so I have no idea of how difficult it is to "teach" a robot what to do next.
Do some reading on the forum, there is an interesting thread about "computer programmers" that are interested in "robot programming"
For the next project we have to repack enumerous products into different packages (little bags or small cartons). How long does it take to programm a robotic arm such a new task
Depends on many factors such as how well your teaching and programming skills are, they layout of the process, the end of arm tooling
and is a robot like the ABB irb 120 flexible enough (concerning the programming work) to repack two days one article and the next two days another article?
If your EOAT allows it, changing to one product to another it could take less than a second, depending and how well structure is your program