What's the history behind the Polish word for Germany/German etc?
There are two hypotheses:
1) The name "Niemiec" comes from the word "niemy" (mute person, one who does not speak) - https://en.wiktionary.org/wiki/Niemiec
2) The name "Niemcy" comes from the name of Germanic tribe "Nemeti" - https://en.wikipedia.org/wiki/Nemetes
Wikipedia's article Names of Germany has an interesting and detailed answer.
The way flaga is pronounced makes it sound like a question. Seems to be a very common inflection issue on duolingo with Polish language.