Hello,
Relations and Pro/Program use a proprietary PTC language (BASIC-like), which is fairly simple and understandable. See this thread for additional info:
https://community.ptc.com/t5/Creo-Modeling-Questions/Pro-programming-user-guide/td-p/323354

If you're looking for more advanced programming and automation, Creo Parametric has APIs for VBA, Java (J-Link) and C/C++. You can find documents related to this subject on PTC's website, but first I would suggest looking at Relations and Pro/Program which might be enough for your scenario (and easier to use).

Regards