Let's say we have a spy that has been True Polymorphed into a sheep (to blend in, obviously). The spy originally knew Common + some other languages. As a sheep, though, she can only bleat.
True Polymorph includes in the description (emphasis mine):
The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
[…]
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech, unless its new form is capable of such actions.
Let's further say that the sheep is spying on some cultists at the local farm. One of the cultists is speaking a language that the spy did not originally know and has cast Tongues on himself to communicate with his bretheren.
Tongues says (emphasis mine):
This spell grants the creature you touch the ability to understand any spoken language it hears. Moreover, when the target speaks, any creature that knows at least one language and can hear the target understands what it says.
Assuming our woolly friend is close enough to hear, does she know what's being said?
More specifically: Can a creature that knows a language, after having been polymorphed into a beast, understand or speak any language at all (Sheep, Cow, Pig, etc.), such that it could understand someone speaking under the effect of Tongues?
Best Answer
No. Languages are part of the game stats replaced by true polymorph
Language is a part of the stat block of a creature, and those are replaced by true polymorph. Unless the new form has the language in its stat block, tongues won't allow it to do so. If the form was something like a tressym it would understand Common, but not speak it.