Okay, I ran
sudo mysql
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '***';
and got a positive feedback
Query OK, 0 rows affected (0.01 sec)
I opened http://localhost/dolibarr/ in Firefox and followed the link to finish the installation process.
Page 3: Still the red sign with the exclamation mark (intl functions).
Page 4: Entered database password and superuser password.
Page 5: New error messages.
(I also got a green tick on “Creation of database (user root): dolibarrdebian”. It is not on the screenshot because I took it the second time I came to page 5.)
So basically this screen gives five possible generic reasons for failure. 
How can I find out what is wrong in my case?
