Actually, it’s never been enforced when Germans openly criticized Bush, Reagan, Thatcher, Putin, Orban (Hungary’s leader), or a dozen-other-odd foreign leaders. This has been brought up in recent days in Germany....that no one can think of a single person that they’ve used this law upon in thirty-odd years.
For this reason, there’s a lot of doubt among Germans (intellectuals and journalists) that any court session will result in a conviction. In fact, if they were to convict him....I suspect at least 50,000 other Germans would openly insult Turkey’s supreme leader and they’d be forced to openly admit they can’t move ahead.
Probably they didn't file a suit, Erdogan has done it.