Okay, if we go your way, both words are Russian; the word "произошло" derives from the verb "идти" (to go), and "случилось" derives from the noun "случай" (an occasion).
Произошло and случилось are synonyms too, but there is a very slight difference between them. For example:
1. Процесс протекал нормально, но внезапно (more likely) произошла ошибка. (A process went normally, but suddenly an error occurred.)
2. Когда учишь язык, ошибки (more likely) случаются. (While learning a language, errors happen).