Disclaimer - This does not matter in the slightest to be honest, 100% intended without a hint of judgement. But in case anyone wanted to know, from someone who lives maybe 4 hours away from Niagara Falls, the way to pronounce it in English is not Nee-a-gar-a, it’s nye-ah-gruh (nye like eye, ah as a soft “A”, potentially nai-a-gruh is easier but to my eyes nye-ah-gruh makes more sense).
If you’re not from the area, even as a native English speaker, you probably pronounce this differently as well, which is completely fine. This absolutely doesn’t matter, it’s a unique word and English makes very little sense as far as any comprehensive rules or consistency, but I come across a lot of people in various videos and whatnot pronouncing it as nee-a-gar-a instead of nye-ah-gruh.
I want to restate that this literally doesn’t matter, the word is a bad translation of potentially several Native American words, the most prominent of which is an Iroquoian word, “Onguiaahra”, so we’re already pronouncing it wrong. English, as I have been told, is one of the hardest languages to learn for non-native speakers, so again absolutely no judgement. I can only speak English as of right now, and am impressed by anyone who can speak more than one language.
TL;DR - This is an FYI -at most-; if you wanted to know the English pronunciation, it’s nye-ah-gruh. Please feel free to ignore this at your discretion.