I found a few mentions in the relatively well known sources:

(Battle of Kosovo, but known in Turkish as Second Battle of Kosovo after 1389) When Sultan Murad conquered Akçahisar in Arnavut (Albania), he was about to allow the army to proceed, and he himself would come to Edirne. However, rumors arrived that the Hungarian infidel was coming with a great army and that his vanguard was passing through Belgrade. Upon hearing this, the Sultan said, "This is Vılkoğlu's (Durad Brankoviç) devilry." And when he added, "Find out who these infidel beys are that are coming," they sent Martalaz Doğan. Martalaz Doğan went, observed the situation, learned the truth, and returned to the Sultan. He reported, "My sovereign sultan! One of these arriving infidel beys is Lih Ban (Polish king), another is Çih Ban (Czech king), one is the son of Lök Ban (Lithuanian king?), and one is Sekületi Ban (?). These are the great sultans of the Hungarian realm. Moreover, each of them is equal in rank to the king. The source of all this mischief is Yanko Hunyad. He is the one who went ahead of them and brought them."

And here is an incredible conversation between the captured Czech king and the Sultan:

Çeh Ban (Czech King) was captured and brought before the Sultan. The Sultan summoned an interpreter and asked, "I have not waged war against you. Why did you attack my land and march against me?"

The infidel responded, "To our eyes, captivity like this seems inevitable." Then he added, "However, I have a few words to say to the Sultan. If he permits, I shall speak."

When the Sultan told him to speak, he said, "First, I will come every year with five thousand elite men to serve the Sultan. Additionally, I will give ten cities, each generating substantial revenues. Let your tax collectors come and collect tribute. Furthermore, let your laws be enforced throughout my land."

The Sultan turned to him and said, "With the help of Allah, I have no need for anything you offer—neither your wealth, nor your fortresses, nor your soldiers."

Then the infidel lord said, "A ruler as noble and wealthy as you deserves whatever misfortune befalls those who come against him."

Upon hearing this, the Sultan drew his sword, handed it to the executioner, and ordered, "Strike off this infidel’s head."

The captive then pleaded, "Mighty Sultan, let me die by your hand. Let me perish at your hands."

Understanding his true intent, the Sultan realized that the prisoner wished to be able to boast even in death, claiming, "A great and renowned Sultan personally executed me."

Recognizing this, the Sultan signaled to the executioner, who struck once, and the captive’s head fell to the ground like a ball.