In 90 days, you can certainly memorize X number of useful words and phrases. If you are actually living in an immersive environment (and you have a reasonably o

In 90 days, you can certainly memorize X number of useful words and phrases. If you are actually living in an immersive environment (and you have a reasonably outgoing personality) you'd probably become pretty fluent within the small domain of that X number of words and phrases. But as for mastering a language, there's no situation, tool or technique that will do more than get you started on your journey within three months. Having foreign words and phrases available in your memory is very different than having those words on your tongue as part of your ingrained language. It's a bit like the difference between off-line tape backup and RAM. Yes the data is there somewhere and you can get to it eventually but you probably can't pull it up fast enough to be useful.