If people are doubting you're japanese, why not just post a vocaroo recording of you reading something japanese. For example, the Louise copypasta?
Or if that's too cringeworthy, perhaps the top of the japanese vtuber wikipedia page?
https://ja.wikipedia.org/wiki/%E3%83%90%E3%83%BC%E3%83%81%E3%83%A3%E3%83%ABYouTuber