Storing dictionaries in mongo
For optimal performance, MongoDB schema design must be custom-tailored for each application's unique access patterns. It's better to study general MongoDB design techniques than specific architectures (which may not match your specific application).

Having said that, Wordnik is a great case-study:


Similar application: real-time dictionary
20 billion records stored in 3.5 terabytes of MongoDB
Many online presentations sharing their MongoDB knowledge/experiences


Inside Wordnik's Architecture gives a more "macro" view of Wordnik's 
architecture.
Lots of other slides



Here is how Wordnik apparently modeled their words:

source: MongoDB Schema Design