Local college uses Rosetta stone to train missionaries and seem pretty happy with the results.
As for retention DirtyDon has it right:
Quoted:
For what it's worth, having been to the Defense Language Institute twice, if you do not use your language skills they will diminish over time regardless of how you learn it (native born speakers "might" be an exception). Total immersion in a language is the only way to go, if you want true proficiency in reading, writing, and speaking.
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BTW, if you don't use it you lose your native language.
Example 1. Friend went to Germany for a year lived with Germans, spoke German 24/7. Came back - fluent in German, forgot some English.
Example 2. Hispanic minister at our church was born in Dominican Republic. I asked him to help me translate a document into Spanish. He couldn't remember some of the words because they weren't part of his regular vocabulary here.