Bad breath comes from a number of places, the two most common being the mouth and the stomach. The neutralizing powers of yogurt and other probiotics treat the latter cause.
Yogurt shouldn’t have any effect at all on the bacteria that live on the tongue because it’s not there long enough. However, it likely has a neutralizing effect on the acid that resides anywhere between the mouth and the stomach, including the back of the throat and the esophagus. Almost any milk product or food that contains live cultures would have the same effect.
While yogurt can get rid of bad breath that results from gastrological conditions, like acid reflux, it won’t have any real effect on bad breath that is the product of gum, liver or lung disease. For chronic bad breath, check with your doctor for probable causes.