21. A system, comprising: a content database containing a plurality of base language texts; an assessment module configured to produce an indication of a user's comprehension of each word from a plurality of foreign language words in a target foreign language, each of the plurality of foreign language words corresponding to a base language word; and a processor configured to substitute a foreign language word from the plurality of foreign language words for a portion of the instances of a corresponding base language word occurring in the base language text to produce a mixed language version of the base language text, the portion based on the indication.
21. A system, comprising: a content database containing a plurality of base language texts; an assessment module configured to produce an indication of a user's comprehension of each word from a plurality of foreign language words in a target foreign language, each of the plurality of foreign language words corresponding to a base language word; and a processor configured to substitute a foreign language word from the plurality of foreign language words for a portion of the instances of a corresponding base language word occurring in the base language text to produce a mixed language version of the base language text, the portion based on the indication. 31. The system of claim 21 , wherein the assessment module is configured to query the user regarding the user's subjective comfort with an amount of target foreign language words in the mixed language version of the base language text presented to the user.