"A" and "An" are English, I personally say it the way the japanese say it because I am referencing a Japanese word.
Any words (when speaking English) beginning with a vowel in the English language, aeiou, you would say "an".
Examples: An apple, an acorn
It would work with banana, "an banana" sounds silly, so just apply it to that if you're speaking English but referencing a Japanese word in an English sentence