I would assume for Romance-language-speaking peoples, the Mass was in Latin because it started in the language of the people and then they didn't want to update the liturgy as the language evolved. But why did they have Mass in Latin for peoples who spoke Germanic or Celtic or Slavic languages? Was this always the case? Would the missionaries who spread Christianity to Germanic or Celtic peoples in the 5th and 6th centuries have held Mass in Latin for their converts?
Why did Latin Mass last so long? Was there a dialogue about having Mass in the language of the people before the Reformation?
The type of Mass said in a place depended on which Patriarchal Church governed or evangelized an area. In the ancient Church (barring questions of the Papacy for the moment) the highest everyday authorities were the Patriarchs (the highest absolute authority was/is an Ecumenical Council). The five ancient Patriarchs were the Bishops of Rome, Constantinople, Antioch, Alexandria, and Jerusalem. Within those territories the same sort of Mass (in the Eastern Church called the Divine Liturgy) was said, with various local variations. Since Europe was evangelized by the Latin Church, the Mass was in Latin. Even as Latin stopped being the spoken language of most people, it did not stop being the language of literate people/literature. Latin was the language which was written, and this helped to continue to have the Mass in Latin.
This issue was further solidified by the Council of Trent. That Council banned any form of Mass (there used to be a great variety of liturgical uses in Europe) which was less than 200 years old. From that moment on all Masses were to be the Missal of 1570 (which was in Latin) or any of the older than 200 year old, suppressed Latin Uses which also were all in Latin.
Despite the foregoing, there were attempts and discussions of having the Mass in Latin before the Council of Trent. These just did not take widespread effect, and were forbidden after Trent. Although Eastern Liturgies are considered to be in the language of the people, before recently they were offered in Old Church Slavonic or old liturgical Greek which is noticeably different from contemporary slavic languages and Greek.
However, there were exceptions to this general practice. Missionaries to China (Jesuits in the 17th Century) were given permission to say the Mass in Chinese. There were also indults to say the Mass in certain Slavic tongues at various times in various places.