1. A method, implemented in a computing system, comprising: receiving a base language text including a plurality of base language words; receiving a proficiency datum associated with a user's comprehension of a target foreign language word corresponding to a base language word from the plurality of base language words; substituting the target foreign language word for a portion of the instances of the corresponding base language word occurring in the base language text to produce a mixed language text, the portion determined based on the proficiency datum, the substituting performed by a processor; and presenting the mixed language text to the user.