There is historical precedence - for example Recep Erdogan addressed followers in 2014 in Cologone/Germany.
The difference is that this was part of a state visit and he spoke with permission of the German government.
The German "Bundesverfassungsgericht" refused to grant a temporary injunction (if that is the right word) to allow Erdogans planned 2017 speech - mainly because the applicant (not Erdogan) could not prove that his own rights were infringed (so he was not entitled to an injunction).
However in the the full statement of the court, which is not yet public, the judges pointed out that Erdogan does not enjoy a fundamental right of free speech, since fundamental rights do only apply to persons, and Erdogan represents the Turkish state (the point of fundamental rights is to protect people against the state, so states themselves cannot have fundamental rights).
Turkey claims a breach of international law, specifically the Vienna Convention on Diplomatic Relations, since it is the job of the diplomatic staff to "protect the interests of their country" (extremely rough translation of a passage in this article of "Der Spiegel"), and apparently they think this includes soliciting votes for their referendum (which is not a completely crazy notion since this a major policy issue for Turkish citizens).
If Turkey wants to go through with this claim this will have to be decided by the International Court of Justice, so your question is as of yet not answerable, but their argument seems a little flimsy.