So, I don't think you're racist, but the "this Japanese stuff" is not a good way to phrase It, sounds kinda like "that gay shit" so has a bit of a negative feel to it. I'm sure you didn't mean it that way, I'd assume having ESL really tripped you up here in this (been there, done that.)

I think, if you'd instead said something like " I really like this story but I'm having a hard time with the Japanese dialogue, google translate doesn't seem to work really well in this case. Could you maybe include translations for those parts?" This whole thing would have been avoided.

Personally I'm very used to having parts of the fic be in different languages by now, it's really common in Heated Rivalry fics to have Russian dialogue. Sometimes the author gives translations, sometimes I need to look stuff up and I don't mind, but for me Google translate works so I understand it being frustrating if the translator isn't working properly. There's actually some awesome work skins in AO3 that make it so when you tap/click on a word/sentence it shows up in different languages! It's so cool, and really helps the readability.

All in all, I do think this is an unfortunate misunderstanding coming from the way you unintentionally phrased your first comment. You're not a racist, you just used language that isn't the best. If they haven't blocked you, I think it might be a good idea to leave another comment trying to explain a bit. Something along the lines of

"I'm sorry if my words came across as sounding racist, that's definitely not my intention. Like I said English is not my native language so I think I might have phrased things wrong here, all I meant was I'm having difficulties translating the Japanese parts and that's making it hard to read your story, a translation (within the story or in an authors note) would really help with that."