I released version 1.0.0 of my Swift library for formatting numbers in Japanese, JapaneseNumberFormatStyle.
It has a single format style implementation that spells integers as Japanese kana. For example:
123.formatted(.japaneseKana) // "ひゃくにじゅうさん"
Or in roman letters, “hyaku nijuu san”.
This library was extracted from my iOS app, Number Tango. It’s a flashcard app for practicing Japanese numbers with audio from a real native speaker. It specifically focuses on recognizing spoken numbers, so you listen to numbers and then type them in (as numerals).