I really like the art, especially the color scheme and the character designs. The villain's mask is pretty awesome, and Frisk is cute and sympathetic. The story seems interesting, but as you mentioned, the English is sometimes lacking, and that can make it a little hard to follow at times. If possible, maybe you should have someone look through the dialogue of each update and correct any mistakes? Maybe ask in the forums if someone would like to do it?
(I haven't played Undertale, so I don't really know the context, sorry.)