Translated by Ralph Anzarouth and an anonymous friend
Loyalty means following the pure truth in all one's dealings with his fellow man, regardless of whether they are with somebody greater than him, with his equals or with someone on a lower level than him. And this is a lofty virtue, which Hashem used to praise the master of prophets1, as it is written (Numbers 12, 7): "My servant Moses is different: he is the most loyal in all my court”.
This virtue is found in the species of bird called “crane”2; they have a king which they serve with a loyalty outstanding among all living creatures. At night, when they want to sleep, they place their king in the center and they surround him; and two or three of them will keep guard and in order that they will not fall asleep during their watch, they stand on one leg, holding a stone with the other, so that if they doze off, the stone will fall and will wake them up. They do all this so as to serve their king faithfully. It is thus fitting that on this higher level man should always serve his king with absolute loyalty. We should therefore behave in this way in all our dealings and conduct all our affairs with loyalty and integrity.
King Solomon said (Proverbs 20, 6): "Many people may be praised for their kindness, but who can find a loyal person?" Our Sages of blessed memory instructed us (Talmud Bavli, treatise Shabbat 31a): "Rava said that the first question man is asked when he comes before the Heavenly Court is if he conducted his affairs with faithfulness etc.". And it is further stated that the destruction of Jerusalem came about only after there ceased to be people of integrity in it, as it is said (Jeremiah 5, 1): "Walk through the streets of Jerusalem etc. and if you find there someone who acts justly and seeks loyalty, I will forgive her".
Seneca said: "One who loses his integrity has nothing more to lose". Aristotle said: "Do not deprive any person of your loyalty, since the lack of it is the trait of harlots and villains". Socrates said: "Be loyal to those who trust you and you will not err. Juvenal said: "All things in the world have supporters and opponents, but loyalty is praised by everybody." Somebody else said that man can achieve greatness by way of three virtues: by being loyal, by speaking the truth and by avoiding despicable thoughts.
It is written in Roman literature that during the Carthaginian wars Marcus Regulus was captured and sent back to Rome to try to arrange an exchange of prisoners. When they gathered for consultation, Marcus Regulus stood up and said that this was not the right thing to do, since the Romans who were prisoners in the hands of the Carthaginians were insignificant and almost all of them were elderly and no longer fit for combat; whereas the Carthaginian prisoners in the hands of the Romans were young, strong and valiant men, among their prominent citizens. As a result of this the Romans acted on his advice and rejected the prisoner exchange. In order to avoid breaking his word, Marcus Regulus returned to Carthage to his captivity and in this way demonstrated his loyalty to his fellow citizens in front of all his captors.
Notes of the translators:
 Moses (Moshe).
 ”Gru” in the original text: cranes belong to the order of the gruiformes.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 22 on loyalty is found at pages 47-48.