Hi, and thank you for your interest!
cesium is being translated using Weblate, a web tool designed to ease translating for both developers and translators.
Translation project for cesium into locale_NL-NL (generated) currently contains 417 strings for translation and is 99.7% complete.
If you would like to contribute to translation of cesium, you need to register on this server.
Once you have activated your account just proceed to the translation section.