Hi, and thank you for your interest!
Zoiper iOS is being translated using Weblate, a web tool designed to ease translating for both developers and translators.
Translation project for Zoiper iOS into Japanese currently contains 1053 strings for translation and is 0.0% complete.
If you would like to contribute to translation of Zoiper iOS, you need to register on this server.
Once you have activated your account just proceed to the translation section.