27. The system of claim 22 , wherein: the assessment module is configured to produce an indication of the user's comprehension of each phrase from a plurality of foreign language phrases, each phrase from the plurality of foreign language phrases corresponding to a base language phrase; and the processor is configured to substitute a foreign language phrase from the plurality of foreign language phrases for an amount of the instances of a corresponding base language phrase occurring in the base language text to produce the mixed language version of the base language text, the amount based on the indication of the user's comprehension of each phrase.
22. The system of claim 21 , further comprising: a user proficiency database containing a record including the indication of the user's comprehension of the foreign language word.