From your first post, I understood that both MySQL belong to you, right?
No, I just have full access to the server I want to restore mediawiki to. The server to back up the database has only phpmyadmin for exporting. That's the main problem. I solved it by telling my provider to make a mysqldump (with that latin1 option) by hand (and they luckily do so). But I think, that to this problem should be referred on an offical "How to backup" page at the mediawiki documentation. Cause surely a lot of people do backup of mediawiki by exporting via phpmyadmin and are not aware of, that if they had to restore the database, there is trouble preprogrammed.