Hi, and thank you for your interest!
Zoiper 5 is being translated using Weblate, a web tool designed to ease translating for both developers and translators.
Translation project for Zoiper 5 into English (United States) currently contains 985 strings for translation and is 100.0% complete.
If you would like to contribute to translation of Zoiper 5, you need to register on this server.
Once you have activated your account just proceed to the translation section.