14. A method, implemented in a computing system, comprising: receiving a base language content piece from a content provider; receiving a request for a mixed language version the content piece from a user; receiving an assessment associated with the user's proficiency with a plurality of foreign language words; and producing the mixed language version of the content piece by a processor, the mixed language version including a first foreign language word from the plurality of the foreign language words substituted for all of the instances of a corresponding first base language word occurring in the base language content piece when the assessment indicates that the user is proficient with the first foreign language word, the mixed language version including a second foreign language word from the plurality of the foreign language words substituted for a portion of the instances of a corresponding second base language word occurring in the base language content piece when the assessment indicates that the user is less than proficient with the second foreign language word.
14. A method, implemented in a computing system, comprising: receiving a base language content piece from a content provider; receiving a request for a mixed language version the content piece from a user; receiving an assessment associated with the user's proficiency with a plurality of foreign language words; and producing the mixed language version of the content piece by a processor, the mixed language version including a first foreign language word from the plurality of the foreign language words substituted for all of the instances of a corresponding first base language word occurring in the base language content piece when the assessment indicates that the user is proficient with the first foreign language word, the mixed language version including a second foreign language word from the plurality of the foreign language words substituted for a portion of the instances of a corresponding second base language word occurring in the base language content piece when the assessment indicates that the user is less than proficient with the second foreign language word. 19. The method of claim 14 , wherein the base language content piece is an audio stream.