Non-catholic here ...
What exactly is a Latin Mass? Is everything spoken in Latin or just certain key parts?
Everything’s in Latin start to finish. Usually though the priest will reread the Gospel and Epistle in English.
Most people have missals with the Latin and English side by side so they can follow along. So you just start to learn Latin by osmosis. And since most of the Mass stays the same week to week, everybody learns the standard responses and pretty much knows what is going on.
For the most part Latin.My church here in Detroit also does a mass in German every fourth week of the month. I have been working on both languages. As if English wasn’t bad enough :)
When people refer to "the Latin Mass," they are almost referring to the pre-Vatican II Mass (also referred to as "the old Mass," the "Tridentine Mass," or the "Traditional Latin Mass") as opposed to the "new Mass" or novus ordo, which was composed in Latin, but is generally celebrated in the vernacular. Texts of both are at the links should you wish to compare them.
The old Mass was always (AFAIK) said in Latin. For Sunday Mass, the Gospel was repeated in the vernacular just before the sermon, which was of course also in the vernacular. In daily Mass, there was no vernacular reading of the Gospel and no sermon.
We learned about the nature, structure and parts of the Mass in school (at least those of us in Catholic school), so we knew basically what was going on, even if we didn't have a Latin-English missal to follow along with.
I’ve been at a Latin Mass. You do as you are bound to do! (as always)