Formal:
* Ek lieg nie. (This is the most straightforward and literal translation.)
* Ek is nie besig om te lieg nie. (This emphasizes the action of lying.)
Informal:
* Ek lieg nie vir jou nie. (This is slightly more casual, adding "for you" at the end.)
* Ek sê die waarheid. (This means "I am telling the truth," which implies you are not lying.)
* Ek maak nie grappies nie. (This means "I am not joking," which implies you are serious and not lying.)
The best way to choose which phrase to use depends on the context of the conversation.