Seems really weird that the same thing - linguistical misunderstandings - is happening twice to the same team in the same tournament. Don't get me wrong, I'd say it's entirely a plausible explanation. "Digga" really is commonplace German youth slang without any negative connotations, it's entirely plausible that this word was used on the pitch in every game of the tournament, and equally plausible that it would be misunderstood by a foreigner.
I think it's really weird then that Chelsea would escalate like this, despite understandable explanations being given, to go as far as give a press statement and demand official investigations over such an absolute nothing-burger of an incident. It's good that they're taking racism seriously and back their players against it, but this should have really been over after a quick conversation between the clubs, maybe a quick google for the term, and then they can have a laugh about it together.