I really admire people like Simon, who is preparing to go as a missionary to Japan, and while doing so, he is allready learning japanese. Most people don't, specially if they are in bible school at the same time.
Learning the language is essencial. Language is the biggest breakthrough. If you can't talk, there is not much you can do. And that process should start before you embark on the plane. We get plenty of missionaries here with no language skills, and they are a blessing, but there's not much they can do, and they get frustrated really fast (yes, even those that arrive smilling from ear to ear).
You should at least know what you are geting into. "Sit down and estimate the cost" (Luke 14:28). Normally people need at least 3 years to learn the language well. And if you are going to a country on a 5-year mission trip, you'll only have 2 years left. Time goes really fast. Some realize portuguese is a hard language too late (~150 different verbal forms? Hundreds of irregular verbs and male/female forms? What's the deal?), and give up.
In the end, being a very important subject, language will come with time. And you will learn it better if you learn in a country where they speak it, no doubt about it. But if you learn the language while you are in bible school, you'll spare yourself much misunderstandings later. And it won't be as easy for the locals to fool you (some "christians" will try to).
In terms of language, be prepared, you'll thank me later.