Informal:
* Mouchard: This is the most common and direct translation of "tattletale." It literally means "snitch" and has a slightly negative connotation.
* Balanceur: This word is more neutral and refers to someone who tells on others, especially to gain an advantage.
* Petit(e) dénonciateur(trice): This translates to "little informer" and carries a strong negative connotation, implying someone who is malicious and petty.
* Raconteur(trice) de bêtises: This means "gossip" and implies someone who spreads rumors and tells on others to cause trouble.
Formal:
* Dénonciateur(trice): This is the formal word for "informer" and can refer to someone who tells on others for serious reasons, such as illegal activity.
* Mouchard: This word can also be used formally, but it's less common and carries a stronger negative connotation.
The best word to use will depend on the specific situation and the tone you want to convey.