An earlier question about a creature that can't speak(a displacer beast) got me thinking: what if the creature was polymorphed into a creature that could understand languages but couldn't speak, such as a pseudodragon?
Polymorph. This spell transforms a creature that you can see within range into a new form.
...
The transformation lasts for the duration
(The duration is 1 hour.)
...
The creature can't ... speak.
The rules for monster game statistics state:
The languages that a monster can speak are listed in alphabetical order. Sometimes a monster can understand a language but can’t speak it, and this is noted in its entry. A “ — ” indicates that a creature neither speaks nor understands any language.
So if the polymorph form’s statblock has a language listed, the creature can understand that language, since the creature’s statistics are replaced by those of the polymorph form:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.
