I tried to make a dialog text engine for scratch based on what griffpatch showed at the end of part 3 of his tutorial series on making a text engine (he said he would show how to make that but he never finished it). The only thing it doesn't have is the ability to reveal the text letter-by-letter.
thanks to @griffpatch fo the text engine tutorial. This does use a list to store the dialog. You can use this if you want (no credit required). Instructions on how to use this are inside the "dialog" sprite. I'll unshare this if griffpatch makes a tutorial for it. If you use this, be sure to import both the dialog and the font sprites (the font sprite is a little bit different from the one in the tutorial)