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 currently contains 961 strings for translation and is being translated into 45 languages. Overall, these translations are 16.4% 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.