Not just in Polish - word Niemiec, Nemec, немец etc. you will find used (not necessarily as the most common word nowadays) in many Slavian languages.

Simply because Slavians share common roots of the language and can more or less communicate with each other while Germanic tribes were 'non-speakers'.

I also read that a lot of native American tribe names came about when early European explorers asked one tribe, "What is the name of the tribe that lives over there?". The local tribe would respond, "We call them the other people" So the Europeans would translate the tribe's vocalization of "Other People" into the name for that tribe.

There are a couple examples of this:

Comanche (from a Ute word meaning "they fight with us.")

Apache (from a Zuni word for "enemy.")

"The term Maliseet is the exonym by which the Mi'kmaq people referred to this group when speaking to early Europeans. Maliseet was a Mi'kmaq word meaning "broken talkers", "lazy speakers" or "he speaks badly," or differently by which the Mi'kmaq contrasted the other tribe's language to their own.[3] The Wolastoqiyik and Mi'kmaq languages are closely related but distinctly different. The Europeans met the Mi'kmaq before the Wolastoqiyik, and adopted their term of Malesse'jik (transliterated as Malécite in French) for the people, not understanding that it was not their true name. The later English colonists anglicized this term as Maliseet, in another transliteration of sound."

This is hilarious. I love the humor of our neighbors to the east. :)