It depends on the context. The reason is because 〜たら does not translate directly to English; it sometimes takes the meaning "if" and sometimes takes the meaning "when".
Without context, simply reading the sentence as stated, it is assumed that Mr. Smith is not in Japan, nor is he planning to go to Japan. Given that Mr. Smith is neither in Japan nor going to Japan, there is no context as to why Mr. Smith needs a Japanese translator at all. Therefore, 〜たら would take the meaning of "if", and the translation would be "Since Mr. Smith doesn't speak Japanese, he will need someone to do accurate translation for him if he goes to Japan", which is close to your translation.
Now, add to that the context that it is highly likely that Mr. Smith will someday go to Japan for a business meeting. This changes the meaning of the sentence. In this case, 〜たら takes the meaning of "when", since it is determined that Mr. Smith is indeed likely going to Japan (although perhaps at some indeterminate point in the future). In this case, since Mr. Smith will be in Japan, and he will have a need to use Japanese while in Japan (in a business meeting with a Japanese person), the translation changes to "Since Mr. Smith doesn't speak Japanese, he will need someone in Japan to do accurate translation for him (when he goes there for his business meeting)".
So, depending on context, both translations can be correct. Of course, in examples from Japanese-learner textbooks, every non-Japanese person is always planning imminent business trips to Japan, which is why they chose the latter case to use, without context; the context is implied since it's a Japanese-learner textbook (I say this tongue-in-cheek, but it's also pretty close to true).