septimus:
IIUC, most scholars concur that the biggest influence on 12th-century English, and thus on the transition from Wessex Old English to London Early Middle English, was not Norman French, but rather the heavily Norsified language of the Danelaw. The influence of (Parisian!) French did not intensify until (paradoxically?) the Norman overlords adopted English as their first language in the early 13th century.
The Norman Conquest *did *serve as a key catalyst for the development of Early Middle English: The people of South England and the people of the Danelaw were rivals but the Norman conquest made them allies (“my enemy’s enemies are my friends”), so a common language (Early M.E.) arose.
But there must be countless other countries with this kind of messy interaction between different ethnicties over the course of several invasions. Why didn’t other languages end up with a ton of synonyms?