Prior to the 2nd Vatican Council, how would people, let's say, from Scandinavia during the introduction of Christianity, gain understanding of the Gospel from Christian Masses held in Latin given that literacy rates were abysmal to read the bible on their own?

The coming of catholicism to northern and eastern europe took place generally (it depended a lot on the region) between 800 AD and 1200 AD.

Generally speaking the regions you are asking about gained catholicism from either peaceful transition such as building churches/chapels in the sacred places of the pagan religion or converting a tribal chief or king or by violent means.

A good concrete example is the passion play mentioned in Henry's Chronicle of Livonia in which the germanic clerics put a play on to impart the story of the passion to the native people.

No "normal" person in these locations could read latin but rather only the clerics and possibly (depends on exact year) the rulers of the countries/area.

There were also clerics who preached the word of god outside of mass and would have one so in the local language or through an interpreter.