A friend of mine is of Lakota descent, and we've discussed this from time to time. Here's what she says, in a nutshell:
The Lakota language really doesn't translate well at ALL into English, because there are many concepts and philosophies among the Lakota, which is considered a "pre-reservation" language, that simply have no English equivalent. Likewise, there are a number of English ideas that don't translate into Lakota. For example. in Lakota culture many of the references are based upon nature, seasonal cycles, and travel. While you might say to someone, "Welcome!" in English, a Lakota would say "I am glad that you've walked so far to get here."
In addition, many language ideas are metaphors. While you might say in English, "Look at those stars up there," a Lakota could say "It fills me with happiness to see something so great in the heavens" -- because the expression is based upon the humility of man in the presence of nature, rather than just a couple of twinkly balls of light.
There are also several different variations of Lakota language, I believe she told me eleven, but I'm not positive on that.
Finally, she tells me that many English concepts just don't work in Lakota. For example, we say "I might go eat dinner." Among the Lakota, you either do or you don't, there's no maybe about it. You might say "I will think about eating dinner," because it makes a definite statement.
I know this didn't answer your original question, but hopefully it will help some