Someone else had the right idea with a Latin/English book. Latin was used by priests and a few literate people throughout Europe. With that you could find refuge in a monastery or nunnery until you learned the local language. I think in England they had Old Norse. I have a 15th century fascimile of a Spanish/English reader. In Spain they have had an academy for centuries to keep the language “pure”. Since I am fairly fluent in Spanish, it was actually easier to read and understand the Spanish than the English.
“I think in England they had Old Norse.”
In 742 it would have been Olde English, since the Norse first raided Lindisfarne (north east England) about 793.
Not to nit pick.
Old English descended from the Anglo-Saxons-Frisians-Jutes who had been going from the continent to Britain since around 500.
The Viking raiders and settlers came later.
Our modern English is the product of old English, Old Norse, Norman French, Latin, Gaelic, etc.
And a fine language it is, with a very large vocabulary making for precise and economic writing and speaking.
Good idea! Or, become fluent in Latin (and Spanish) before going.