30. The system of claim 21 , wherein the assessment module is configured to present to the user a plurality of questions and answers regarding the content of a passage of text including at least the foreign language word.
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.