It is important for a people group to possess God's Word in their own language so they can understand it on a deep level. If many of the tribal people speak the trade language, they may be able to understand the Bible in that language. But if it is not their heart language, the one they use and understand best, it may be difficult for them to understand deeper things. Let me try to make a more personal example. Maybe you studied Spanish in high school or have been to Mexico. You may even feel like you can "get around" in the marketplace. However, you would probably be intimated if someone asked you to pray in Spanish or preach a sermon, because those require a much better understanding of the language. It is the same way for a tribal person who tries to read the Scriptures in a language that is not his heart language.
Now that I've established a rationale for why translation is necessary, I want to share a couple of things I learned in the class called "Semantics and Translation". Word for word translations are not as word for word as we might think. Sometimes the translators must change word order to make it have the correct meaning in English or they are forced to use more than one word to represent the meaning of a single Greek word. There are many complexities that exist in transferring the Scriptures into a foreign language. For instance, what if they don't have certain key words, like: love, sin, or forgive? In the end, the question really comes down to this...which is more important to preserve in your translation? The original form or the meaning it conveyed? I would like to argue that the meaning is more important. The form cannot be directly transferred from one language to another (and still make sense) without changing a little.
Hopefully this provided you with a little sneak peek into the need for and complexities of translation work. It is a huge task, but so necessary, and definitely rewarding when a tribal person is able to read Scripture in his very own language.