I accept "mangoes" because of the English rule that nouns that end in a consonant followed by a vowel should be padded with another vowel ("e") before the "-s". Another example I can think of off the top of my head is "heroes", not "heros".
However, I also accept "mangos" because it feels right. Wiktionary says "mango" is Portuguese, and I don't know Portuguese, but at least in Spanish you don't pluralize with "-es", just "-s".
My bad. Yeah, it's interesting that we don't seem to know why English calls it "mango" when every other language calls it "manga" or something else ending with the "-a".
A lot of words in English are mispronounced/misheard/misunderstood words of things that people from the UK didn't recognize during their exploration eras, so they asked the locals and that's what they told everyone it was. Eventually Aussies and Americans inherited these "mistakes".
By the time the error was found out everyone was calling it by the wrong name already, so it kinda stuck. Hell i speak Portuguese and when i speak in English i tend to say mangoes instead of mangas, even though the word mango came from the language i speak.