I’ve just added a note to one of my transactions via the API containing some Kanji. I’m happy to say that everything just worked.
There’s no reason I should have a problem, but non-English characters do often cause problems for other companies so I’m happy to see that Monzo is different.
Same here, I’m pretty happy that I can send internal Monzo payments with Japanese text and full on emoticons. The iOS app even generates proper Monzo.me links for just about every Unicode string I’ve thrown at it!