Well . . . yes and no. There is a lot of research on how immersion can affect language acquisition and the general thought at this point is, yes, immersion can help in language learning, as long as there is sufficient familiarity between the learner's mother tongue (or L1) and the language being learned.

Essentially, if a language is too dissimilar to your first language, then being immersed in the culture can actually limit one's learning because it can be overwhelming. This is one reason why it can be really difficult for say, English speakers to learn to speak Cantonese fluently and why it's common to encounter various immigrants in the U.S. who may have lived in the country for a long time, but still speak using grammatical constructions similar to those found in their L1.

One reason why language classes can be good, is that a capable language teacher will be able to help break students of their tendency to form sentences in their L1 and then translate them word for word into their target language (L2). This might not seem to be a problem, but it's a bit like when kids learn to read. Most people will subvocalize at least a little bit when reading and this is actually the greatest obstacle to reading quickly. It's easier to tell with kids, because they may be reading silently, but their lips might be moving. Anyway, it's like running two programs on a computer at the same time, generating responses from one program and then pasting them into the second program and hoping that everything will come out right, knowing full well that the second program operates along different parameters.