So in the dream bubbles, as well as in Fefetasprite, Feferi and Nepeta don't appear to be able to speak using actual words, but only with their respective faces. Is there a reason for this?

It’s a joke. Both characters hadn’t appeared in a long time and many fans wanted them to. So Hussie gave us the characters but didn’t make them speak leaving the fans’ desires for the characters partially unfulfilled. 

In short, Hussie the pwnmastr trololololed al us n00bs XD