Hi, and thank you for your interest!
Freeplane is being translated using Weblate, a web tool designed to ease translating for both developers and translators.
Translation project for Freeplane into Arabic currently contains 2049 strings for translation and is 17.9% complete.
If you would like to contribute to translation of Freeplane, you need to register on this server.
Once you have activated your account just proceed to the translation section.