What was the language used in many Western European church services?

The story behind the answer

It was the liturgical language of the Roman Catholic Church in the West, ensuring uniform worship across kingdoms that spoke many different tongues. Clergy were trained to read scripture in the Vulgate and to celebrate the Mass and sacraments in this shared tongue, which helped maintain doctrinal consistency and a sense of unity from Ireland to Italy.

Beyond church services, it served as the lingua franca of medieval scholarship, law, and diplomacy. Monastic scriptoria, universities, and theologians like Thomas Aquinas wrote in it; Gregorian chant and much sacred music were composed to its texts. While Greek dominated the Eastern churches, Western vernaculars entered worship widely only after the Reformation and, much later, with modern reforms.