NLTimes.nl (Janine Pieters) reported last week:

In one email he wrote: "Erdogan, you fucking goat fucker, burn in hell." An attachment added to the email showed Hitler and Erdogan, with a swastika between them and the text: "There's no difference". In another email he said: "Erdogan takes the whole world hostage with his pernicious ideas. More than seventy years ago, a similar dictator made exactly the same mistake. I regret the Turkish people with this idiot, you do not deserve better." According to the newspaper, the Dutch man is facing charges of insult, insulting an official, and insulting a friendly head of state.

Hurriyet Daily News [Turkey] had a similar story.

I should note that American law was sometimes seen as authorizing similar prosecutions back in the day under the rubric of criminal libel law, though usually by states rather than by the federal government. Truth was not an absolute defense, and the rationale for the prosecutions was often that such statements insult foreign countries and thus threaten to damage peaceful relations with other countries; for a mix of quotes on the subject as of 1887, see Wharton's Digest of the International Law of the United States.

A prominent early opinion supporting such speech restrictions came in a grand jury charge by Pennsylvania Supreme Court Justice Thomas McKean, aimed at William Cobbett, who was accused of libeling the King of Spain and the Spanish ambassador (Spain was then in the pocket of Napoleon's France):

At a time when misunderstandings prevail between the Republic of France and the United States, and when our General Government have appointed public ministers to endeavour to effect their removal and restore the former harmony, some of the journals or newspapers in the city of Philadelphia have teemed with the most irritating invectives, couched in the most vulgar and opprobri­ous language, not only against, the French nation and their allies, but the very met in power with whom the ministers of our country are sent to negociate. These publications have an evident tendency not only to frustrate a recon­ciliation, but to create a rupture and provoke a war between the sister Repub­lics, and seem calculated to vilify—nay, to subvert,—all republican Governments whatsoever. Impressed with the duties of my station, I have used some endeavours for checking these evils, by binding over the editor and printer of one of them—licentious and virulent beyond all former example—to his good behaviour; but he still perseveres in his nefarious publications. He has ransacked our language for terms of reproach and insult, and for the basest accusations against every ruler and distinguished character in France and Spain, with whom we chance to have any intercourse, which it is scarce in nature to forgive. In brief, he braves his recognizance and the laws. It is now with you, gentlemen of the grand jury, to animadvert on his conduct; without your aid it cannot be cor­rected. The Government that will not discountenance, may be thought to adopt it, and be deemed justly chargeable with all the consequences. Every nation ought to avoid giving any real offence to another. Some me­dals and dull jests are mentioned and represented as a ground of quarrel be­tween the English and Dutch in 1672, and likewise caused Louis the XIV to make an expedition into the United Provinces of the Netherlands in the same year, and nearly ruined that Commonwealth. We are sorry to find that our endeavours in this way have not been attended with all the good effects that were expected from them; however we are de­termined to pursue the prevailing vice of the times with zeal and indignation, that crimes may no longer appear less odious for being fashionable, nor the more secure from punishment for being popular.

The grand jury declined to indict Cobbett, but McKean had required Cobbett to put up a bond, and there were complicated further legal proceedings in Pennsylvania as a result; Wharton in the 1870s seemed to think that McKean's view was legally sound, and indeed in 1909 one Carlo de Fornaro, apparently a prominent cartoonist, was convicted of libel for his harsh criticisms of a prominent Mexican politician, delivered in the course of a general criticism of Mexican leader Porfirio Diaz.

I'm pleased to say, though, that American law has turned sharply against attempts to punish speech that insults or even defames foreign political leaders; the old libel rules wouldn't survive New York Times Co. v. Sullivan (1964) and later cases. A foreign leader could presumably still bring normal civil libel suits, but would have to show not just insults but false statements of fact said with knowledge that they were likely false. (In theory, the states that still recognize criminal libel law could also bring criminal prosecutions in similar situations, with the same required showing, though they would also have to prove the case beyond a reasonable doubt; but I know of no successful prosecutions related to libels of high foreign officials—or of high American officials—n the last 50 years.) I'm sorry to see that the Netherlands' view is less protective of its citizens' speech.