I would not recommend this sketch unless someone is desperately searching for a good example of bad coding... 
- unnecessary code repetitions
- "magic" pin numbers
- awkward button functions
- while loop in loop()
- double local declaration of boolean button3IsUp in loop()
Surprisingly the I2C pins for different Arduino controllers are mentioned in a table in the French original link ...
Could be done much more readable and maintainable with a state machine and clear pin and function names ...
Too late today for me, but any volunteers?
ec2021