No non-Latin prayers in European churches before Reformation? Is this true that before the Reformation, all Christians in Europe would have to study Latin in order to understand the prayers pronounced during Masses in Roman Catholic Churches?
No non-Latin prayers in European Catholic Churches before Reformation?

Is this true that before the Reformation, all Christians in Europe would have to study Latin in order to understand the prayers pronounced during Masses in Roman Catholic Churches?

The short answer is no, for two reasons.
The first reason is quite simple. A large percentage of the faithful could not study Latin because they were illiterate.

The Reformation dawned on European societies where literacy was restricted. At the end of the Middle Ages, the ability to write was restricted to less than 10% of men and hardly any women possessed it. This fundamental division between the sexes persisted for centuries and throughout the early modern period male achievements almost always overshadowed those by women. For example, one bridegroom in three could not sign Amsterdam's marriage register in 1630 compared with two-thirds of brides. Until the eighteenth century the rate of improvement for men generally exceeded that for women, so that the gap between the sexes seldom narrowed. Amsterdam brides and grooms both saw illiteracy fall between 1680 and 1780: males from 30% illiteracy to 15% and females from 56% to 36%.14 The literacy of townspeople also grew more quickly than that of rural dwellers and the wealthier merchants and professionals inhabited city centers where concentrations of reading and writing ability were especially high, distinguishing them even from suburban dwellers. By the mid-eighteenth century London and Paris had literacy levels of over 90%, which would not be achieved nationally until the late nineteenth century. In Eastern and Southern Europe nearly the only literate people were town dwellers and rural landowners. - Introduction: The Literacies of Early Modern Europe

The second reason is completely different. Although the Mass of the Roman Rite was Latin in churches of the Roman Catholic Church, exceptions were permitted by Rome.

The Roman Rite is used in Dalmatia in an Old Slavonic version (written in Glagolitic letters), occasionally in Greek in Italy; but in any language it is always the Roman Rite. - Catholic Encyclopedia

In other words, the exact same Mass could be said in at least three languages: Latin, Greek and Old Slavonic and thus were considered Roman Catholic and not Eastern Rite Catholics. Please note that some authors employ the term Greek Rite or Old Slavonic Rite, but in this case, the term means a variation or usage of the Roman Rite.