It's very difficult to answer this question even with just a year. You need to consider geography and social class as well. Multi-lingualism, whether native or learned, tends to depend on a number of sociological factors, such as presence of linguistic communities, interaction between language communities, education and ideas of education, etc..

To look at the issue in reverse, you get widespread monolingualism when there are insufficient factors to cause people to learn/or to grow with a necessary level of exposure to, multiple languages. That means things like:

• the prestige of one language severely outstrips rivals
• there is little social or economic advantage in other languages
• education in other languages is unavailable, insufficient, or not socially esteemed
• people can achieve all their socio-economic needs with a single language
• minority language communities have little widespread interaction with dominant communities (ghettoisation and the like)

I think, for a historical case study, you are going to need to propose a geographical location as well as a year.