That’s a great idea!! 
(Undertale is awesome) I’d suggest starting with a smaller time commitment (Like just making the start of undertale, so it takes a month rather than two years) that way you can update on your progress and get feedback! 
However, I think it would be smart to code the game in either Blocks or JavaScript (because converting between the two often causes a lot of bugs)
I’d also recommend checking out some of the other Undertale games in Makecode!