Firstly, this app has been specially created with people who not only are big fan of Apple Watch but also plan to visit Japan or are interested in Japanese-number or Kanji-number letters in particular.
Secondly, if you dropped by a Japanese-style restaurant in Japan, such as Izakaya (Japanese-style pub), Ryo-tei (Japanese restaurant), buckwheat noodle restaurant, you would probably see some prices on a menu that is written in Kanji number or Japanese-number letters. Then, this app would come in hoping to help you to read how much it is naturally. The app covers some letters of number, such as [one] to [ten], [hundred], [thousand], which are basic Japanese-number or Kanji-number letters, and shows how to read and write.
Thirdly, there are two ways to learn Japanese-number or Kanji-number letters on the app. The one is using [Number List] button, on which you can choose one of Number letters from the list. The other is using [Find Number] button, on which you can set a number between 1-digit and 4-digit, then you can make sure how to write the number you set using Japanese-number letters. As you use the app, you would probably notice to associate Japanese-number or Kanji-number letters from one to ten with English numbers from one to ten respectively though; there is not any Japanese-number or Kanji-number letters associating with eleven, for example. So, if you wanted to write eleven in Japanese-number letters, you should use two letters [ten] and [one], because you could describe eleven saying ten plus one. Following this rule, 21 is written in Japanese-number letters using [two], [ten] and [one], because two times ten plus one is 21.
Lastly, I would like you to feel like having a bit of Japanese-number letters up your Apple Watch, as the saying goes, "have something up ones sleeve."