My understanding is that Latin had become the official language of the mass by around 600 CE, that mass was standardized into the Tridentine Mass in 1570, and that this continued until Vatican II in 1962.
As Vulgar Latin differentiated into the various romance languages, did the mass transition from being understandable to the broad swath of the laity? If so, was doctrine transmitted to the laity in some other manner? Would the typical mass have contained anything that was understandable to someone who did not speak Latin?
I am given to understand that the very well educated would have known Latin as well as their native tongue. I am guessing that the vast majority of the population would not have understood Latin, but is this assumption correct?
I cannot speak to how much Latin Catholics would have been able to understand.
Regarding doctrine though, Mass is generally not for dissemination of doctrine for Catholics, it is a celebration of the eucharistic sacrament. Doctrine in the US was largely disseminated via various education initiatives and from the late 1800's until the 1960's it was largely based on the Baltimore Catechism. Here is a link to the 1941 version. It is in English and has a question and answer format. Here is its explanation of what the Mass is. Many doctrinal issues were also covered by papal encyclicals and other papal documents, most of which have an English version. Here is the Vatican's offerings of Pius XII's writings and speeches, you can use the left navigation to find selections from his papacy on a number of issues. Here is John XXIII's site. Things like the Baltimore Catechism and papal documents would have supplemented and underpinned catechetical classes (largely given to children, but also to adults as well--especially converts), all of which would have been in English. So for teaching doctrine Latin comprehension would have been largely unimportant.
Given that a comment by another participant clarifying that while the Catholic liturgy was in Latin, homilies were given in the vernacular was deleted I will provide a source.
http://www.hcl.harvard.edu/libraries/houghton/collections/early_manuscripts/preaching/language.cfm