Medibang user here! Yes, it has materials with speech bubbles that you can import into the canvas, but I prefer to use the ellipse snap and then I draw the tail of it.
As for fonts, Medibang already has fonts on the cloud, but just in case you don't want to download it, you can use Blambot, which is where I got the font for the main logo of my comic, and it has a lot of dialogue fons that are free.