Anecdata: I've had this happen to me. In travel for an old job, I've had the pleasure of spending time in other countries with foreign languages. Any prep study in those languages absolutely paled in comparison with being forced to use the language all day long. It was especially hard when I was tired from a long day of work. However, I now know that was the most important time for me. Those tired moments when I just wanted wine or beer or a taxi ride and didn't want to speak the language anymore but had no other choice were when I really cemented the language in my head. It's incredibly difficult, and expensive, but I really feel that living in the language for a few weeks is worth the effort and time, much more so than any course. That said, take the course for a few months before you get on the ground, it really helps.