1.Each group's story is important in the history of the Americas. However, it is perhaps the tribes of the center part of the United States that are most recognized.
从however开始到结束，应该是一个强调句，但怎样译出它的意思呢？
这样译如何？（每一部落的故事对于美国历史都是重要的，但是，也许就是美利坚合纵国中心区域的部落是最被人了解的。

2。But we leave the Shoshoni without him adding another one.
这句的译意是不是：但是我们离开shoshoni人，没有加一个shoshoni人。
without him adding another one.这一个短句真让人费解。

your assessment of sentence 1 is correct. 美国中部的部落是众所周知的，最有名的。

as for #2, you need to put in context. "His arms have been painted with long lines. We learn that each line represents a battle. There are many lines. But we leave the Shoshoni without him adding another one. "

他的胳臂上画着长长的线条。我们了解到每条线代表一场战争。（胳臂上）画着许多线条。但是我们离开了他，他并没有添上另外一条（线）。 意思是他们没有打起来。