What is the meaning of "Mah Yiten Lecha"?
Rashi: What will Hashem give to you, the tongue [to stop you from slandering]?
Rashi (4, from Bereishis Rabah 98:23): What will Hashem decree about you in the end? Refer to 120:4:1:2.
Radak: The slander of those who make false accusations against Yisrael in Galus - what will it help you?
Malbim: What benefit do you get from your Leshon ha'Ra? (When the Chayos ask the snake, what Hana'ah do you get from killing, it answers, what Hana'ah does a Ba'al Leshon ha'Ra get? - Erchin 15b).
What is the question "u'Mah Yosif Lecha"?
Rashi and Radak, from Erchin 15b): What guarding and walls can He add for you (the tongue)? You are [already] within two walls (the teeth and lips)! All other limbs are erect and external, and you lie down inside walls of bone and flesh, and still you speak evil!
Radak: Refer to 120:3:1:3.
Malbim: What will your Leshon ha'Ra add to what you already have?