en_tn_lite_do_not_use/mat/12/34.md

You offspring of vipers

Here "offspring" means "having the characteristic of." Vipers are poisonous snakes that are dangerous and represent evil. See how you translated a similar phrase in Matthew 3:7. (See: rc://en/ta/man/translate/figs-metaphor)

You ... you

These are plural and refer to the Pharisees. (See: rc://en/ta/man/translate/figs-you)

how can you say good things?

Jesus uses a question to rebuke the Pharisees. AT: "you cannot say good things." or "you can only say evil things." (See: rc://en/ta/man/translate/figs-rquestion)

out of the abundance of the heart his mouth speaks

Here "heart" is a metonym for the thoughts in a person's mind. Here "mouth" is a synecdoche that represents a person as a whole. AT: "what a person says with his mouth reveals what is in his mind" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-synecdoche)