UNDERTALE Engine is committed to simplifying the process of making an UNDERTALE fangame. None of the codes was came from the original UNDERTALE. It is based on GameMaker Studio 2, and almost everything will be developed on its latest version.
This engine includes: Battle, Overworld, Multi-language supports, and other features. Some of them are even open-source!
If you like this project, please star it and follow it on GameJolt!
We also have a document page, but it is not finished yet.
There is a example branch. Once you have downloaded it and opened it, you can click the "Views" button on the resources panel, and switch the views to checkout different examples.