There was a question recently that was sadly deleted by the OP before I could respond. The question was:

“The 12th century Arab Muslim author and diplomat ibn Munqidh writes extensively about his dealings and even friendships with Franks in the Crusader Kingdom of Jerusalem, what language would they have communicated in?"

Usama ibn Munqidh was a poet, author, and diplomat, and often acted as an ambassador to the Frankish crusaders on behalf of various rulers of Egypt and Syria, especially the emir of Damascus Mu’in ad-Din Unur. Despite sometimes making friends with individual crusaders, he mostly depicts the Franks as unintelligent and uninteresting barbarians from the cultural backwater of Western Europe, and he claims that he never learned their language, “al-Ifranji.”

“They only speak Frankish and we do not understand what they say.”

But he often mentions having conversations with Franks, whether they were random knights or members of the Templar order or even the kings of Jerusalem. If he didn’t speak French, did they communicate with him in Arabic, or did they use interpreters?

For example, Usama briefly lived in Egypt before decided to return home to Syria. He was given letters of safe-conduct from the Frankish king and even sailed on a Frankish ship, which was wrecked along the way - according to Usama, it was because the Franks attacked and destroyed it. In this case Usama was not on board the ship but one of his servants talked to the Frankish king:

“‘My lord king, is this not your document of safe-passage?’ ‘Indeed it is,’ he said. ‘But this is the procedure among the Muslims: if one of their ships is wrecked off one of their towns, then the inhabitants of that town get to pillage it.’ My servant then asked, ‘So you are going to take us prisoner?’ ‘No,’ the king replied, and he had my family (may God curse him) brought to a building, where he had the women searched and took everything they had with them…The Franks took it all and then sent my household five hundred dinars, saying, ‘You can get to your country on this.’”

Usama’s family made it back to Syria safely but they never recovered their property, including Usama’s library of 4000 books.

On another occasion Usama himself complained to the king about a knight, the lord of Banias, who was raiding the lands of Usama’s family in nearby Shaizar. Usama explained the situation and submitted his official complaint:

“I said to the king, Fulk, son of Fulk, ‘This man has encroached upon our rights and seized our flocks right at the time of lambing. But they gave birth and the lambs died, so he returned them to us after so many lambs were lost.’ Then the king turned to six or seven knights: ‘Arise and render a judgment for him.’ So they left his audience-chamber, sequestering themselves and deliberating until their minds were all agreed upon one decision, and then they returned to the king’s audience-chamber. ‘We have passed judgment’, they said, ‘to the effect that the lord of Banias should pay compensation equal to the value of the lambs that were lost from their flock of sheep.’ And so the king ordered him to pay compensation. He entreated me and begged and pleaded with me until I accepted from him four hundred dinars.”

Afterwards Usama and King Fulk spoke to each other:

"The king said to yours truly, ‘By the truth of my religion, I was made very happy indeed yesterday!’ ‘May God keep the king ever joyful!’ I said. ‘What was it that led to your happiness?’ ‘They told me you were a great knight,’ he replied, ‘but I hadn’t really believed it.’ ‘My lord,’ I assured him, ‘I am a knight of my race and people.’"

While Usama was on an embassy to the Franks, he often tried to ransom Muslim prisoners and slaves, which sometimes involved negotiating with the king or another Frankish lord. One one occasion he tried to buy a few slaves from a Frank named William “Jiba” (the Arabic spelling is ambiguous, so we’re not really sure what his French name was).

“I then rode to Jiba’s home (may God curse him) and asked him, ‘Will you sell me ten of them?’ ‘By the truth of my religion,’ he replied, ‘I’ll only sell all of them together.’ ‘I don’t have enough on me to buy all of them together,’ I said. ‘I’ll buy some of them now. At the next opportunity, I’ll buy the rest.’ He merely replied, ‘I’ll only sell all of them together!’”

In the end, all of the slaves managed to escape - possibly with Usama’s help, although he conveniently leaves out the details.

Usama also befriended some Templars, who let him pray in a small mosque beside the al-Aqsa Mosque, which they had transformed into their headquarters:

“One day, I went into the little mosque, recited the opening formula ‘God is great!’ and stood up in prayer. At this, one of the Franks rushed at me and grabbed me and turned my face towards the east, saying, “Pray like this!’ A group of Templars hurried towards him, took hold of the Frank and took him away from me. I then returned to my prayers. The Frank, that very same one, took advantage of their inattention and returned, rushing upon me and turning my face to the east, saying, ‘Pray like this!’ So the Templars came in again, grabbed him and threw him out. They apologized to me, saying, ‘This man is a stranger, just arrived from the Frankish lands sometime in the past few days. He has never before seen anyone who did not pray towards the east.’”

Another newcomer to the east ended up befriending Usama and called him his “brother”:

“When he resolved to take to the sea back to his country, he said to me: ‘My brother, I am leaving for my country. I want you to send your son (my son, who was with me, was fourteen years old) with me to my country, where he can observe the knights and acquire reason and chivalry. When he returns, he will be like a truly rational man.’”

Of course Usama thought that was completely bizarre - what could these barbarians possibly teach a Muslim child? He didn’t say so though, and the knight was satisfied with Usama’s more polite excuse - Usama’s mother would miss her grandson too much if he left.

He sometimes included French words in his book, transliterated and translated into Arabic, so he at least understood a few words. For example he knew “la dame”, which he transliterated as “al-dama” and translated as “al-sitt”, “the lady”. (This is part of a story, probably a joke, about a knight and his wife who have their pubic hair shaved by a barber in the public baths, which I wrote about in a previous answer. In another story he talks about the role of the “viscount”, a legal official who represented the king in cities outside of Jerusalem. He recorded the word as “al-biskund” and translated it as “shihna”, the equivalent legal official he was familiar with in Muslim cities like Damascus. But knowing a few words is far from being able to have a conversation in French.

It seems unlikely that any of the kings of Jerusalem were fluent in Arabic, or that the Templars or recent arrivals from Europe knew any Arabic. But some Franks were known to have learned it. For example, in 1187, when Saladin re-conquered most of the kingdom, Reginald of Sidon told his troops to surrender Sidon in Arabic, so Saladin would understand him, and then told them to keep fighting in French, which Saladin didn’t understand. Another Frankish noble, Humphrey of Toron, acted as an interpreter and ambassador between Richard the Lionheart and Saladin during the Third Crusade a few years later. However, they seem to be exceptions. Even someone like Reynald of Chatillon, who had spent 17 years in Muslim prisons, never bothered to learn Arabic, and had to use an interpreter when he was taken prisoner by Saladin at the Battle of Hattin in 1187. According to Saladin’s secretary Baha ad-Din ibn Shaddad, Saladin:

“summoned King Guy, his brother [Aimery of Lusignan] and Prince Reynald. He handed the king a drink of iced julep, from which he drank, being dreadfully thirsty, and he then passed some of it to Prince Reynald. The sultan said to the interpreter, ‘Tell the King, ‘You are the one giving him a drink. I have not given him any drink.’”

Saladin later had Reynald executed in front of Guy because of Reynald’s past treachery against Muslim caravans and pilgrims, but assured Guy that he would be spared, as a fellow king whom he had captured in battle.