Who would have a good or fair command of Latin in medieval Western Europe?

This is an interesting conjecture. I would imagine that anyone who had contact with the church would know a little bit of it. As previously stated, the devil is in the details. Did clergymen know it to the conversational level or did they just know enough for their role? I would also imagine that most scientist/ alchemists/ physicians would know more than the average nobleman due to their "research". I also give the average layman more credit than others. I believe that the common man would know more than given credit for. I would imagine that living in an bilingual environment would almost create a slang in everyday life

Can't cite a specific episode, but the British History podcast quotes several pre-Carolingian religious figures decrying the fact that nobody can understand the scriptures. At various points then Latin was as dead as it is now.

I'm not sure it was a "conversational" language - it was the best option if you didn't share a common language.

This is an example of My Professional Historian Girlfriend's first law, "It's a little more complicated than that...."

Newton and his contemporaries I think all had Latin. This is a primary effect of the education system of his and earlier times; math and physics were secondary in education. Even in Turing's time, his interest in math and chemistry was not apparently well regarded. He should have been spending more time on Latin and Greek, according to his instructors.

The main and central institution that would have had a "good command" of Latin during the Middle Ages........was The Roman Catholic Church.

The entire Vatican bureaucracy-(from the Pope, to Cardinals, to Monks, as well as to Priests in numerous Churches throughout Western and Northern Europe), would have conducted the Mass exclusively in Latin. The Church attendees-(i.e. the Laity),would have had a "fair command" of the Latin language. Though most Medieval Europeans during the Early Middle Ages-(i.e. "The Dark Ages", 476 AD/CE-1050 AD/CE) were illiterate, their knowledge and command of Latin would have been exclusively verbal/oral.

During the Late Middle Ages-(1050 AD/CE-1400 AD/CE), "The Age of Scholasticism" was ushered in primarily by the Roman Church. Although prominent Early Medieval figures, such as Saint Benedict and especially, Charlemagne, valued and championed the importance of scholarship and literacy, their efforts were, ultimately, short-lived and obscured by the larger Dark Ages.

However, centuries later, at the beginning of the 2nd millennium AD/CE, Medieval Western and Northern Europe began to expand the study of religion into Theology and other similar subjects were studied as a way of complimenting and reinforcing the value of Theology. The Medieval Theological University was born, in countries, such as England, Germany, France and especially, Italy-(North of Rome). It was the Late Medieval European Theological University that actually furthered and advanced the intellectual use of Latin-(since Roman times). The entire University Bureaucracy-(From the Provost, to the Dean, to the Professor, to the Lecturer, to the Scribe and to the student), were well versed in Advanced Latin.

Anyone with an advanced education in either the secular or religious fields would know Latin. It was what most literature taught was written in, and all the religious texts.

Bishop Asser tells the story of how as a child Alfred [the Great of England] won as a prize a book of Saxon poems, offered by his mother to the first of her children able to memorize it.[1]

https://en.wikipedia.org/wiki/Alfred_the_Great#Childhood2

Following the example of Charlemagne, Alfred [the Great of England] established a court school for the education of his own children, those of the nobility, and "a good many of lesser birth".[92] There they studied books in both English and Latin and "devoted themselves to writing, to such an extent .... they were seen to be devoted and intelligent students of the liberal arts".[104]

https://en.wikipedia.org/wiki/Alfred_the_Great#Religion_and_culture[1]

Charlemagne instituted schools to educate members of the nobility.

Charlemagne's native language was probably Old High German.

He also spoke Latin and understood Greek, according to Einhard (Grecam vero melius intellegere quam pronuntiare poterat, "he could understand Greek better than he could speak it").[104]

https://en.wikipedia.org/wiki/Charlemagne#Writing_reforms2

there is a story that he practiced writing but keep his writings hidden under his pillow because of embaressment at his poor style.

Chilperic I c. 539-584 was a Frankish king with some degree of culture.

Yet, he was also a man of culture: he was a musician of some talent, and he wrote verse (modelled on that of Sedulius); he attempted to reform the Frankish alphabet; and he worked to reduce the worst effects of Salic law upon women.

https://en.wikipedia.org/wiki/Chilperic_I3

I believe Chilperic introduced three new letters to the alphabet that were abolished after his death.

Emperor Frederick II (1194-1250) wrote De arte venandi cum avibus about hunting with falcons.

https://en.wikipedia.org/wiki/De_arte_venandi_cum_avibus4

Conradin (1252-1268) was said to be "beautiful as Absalom, and spoke good Latin".

https://en.wikipedia.org/wiki/Conradin[1]

I believe that William of Norwich (1132-1144) was said to have been taught to read and write (English or Latin?) by his parents. So the author of his biography considered it plausible for ordinary townspeople to have some education - though perhaps not knowledge of Latin.

Any way these example show that Latin speaking and/or literacy as not limited to priests but at least some secular leaders knew Latin.

Added 05-15-2017. According to Dominic Mancini King Edward V (1470-1483?) as literate in at least one language.

In word and deed he gave so many proofs of his liberal education, of polite nay rather scholarly, attainments far beyond his age; ... his special knowledge of literature ... enabled him to discourse elegantly, to understand fully, and to declaim most excellently from any work whether in verse or prose that came into his hands, unless it were from the more abstruse authors. He had such dignity in his whole person, and in his face such charm, that however much they might gaze, he never wearied the eyes of beholders

https://en.wikipedia.org/wiki/Edward_V_of_England5

And that probably included Latin literature as well as English.