While many words in each of the languages that are available in the application are interchangeable, there are many that simply are not and do not compute as replacements for each other. I was thinking a feature of the application could be to return old words into circulation when they are falsely applied to the wrong circumstance. This would - probably - result in enhanced learning of the contextual assets of a target language and make people like me focus just a little bit more on the sentences that the words are placed in rather than just the word being promoted.
Example: I used mismo instead of igual. While both have the same English translation, the context that they are used in is critical to their application. When such an error happens, the improperly used word should be brought back into the repeat stack to help the user understand what the exact word is used for... perhaps with a little popup that says its been added back and to pay attention.