Has dog been drinking water OK? Is it eating? Dog could be dehydrated, could have worms, etc. If the dog has worms or fleas, it could make the dog anemic. The dog needs to be checked by a vet immediately. If it is anemic, it could kill the dog. The age of the dog is also a concern. Only a vet will know AFTER the dog is checked by an exam. Does the dog sleep a lot, or is the dog still very active. Has the dog pooped in the past 24 hours, what about peed? All things your vet will want to know, as well as other questions he's trained to ask.
Decreased circulation, either locally or systemically, and low red blood cell count are the most common causes of pale tongue, gums, and oral mucosa. Either way this causes decreased oxygenation of the tissues. This dog needs veterinary medical attention to determine the cause of the pale tongue. This can be secondary to several serious diseases.
My dog has developed bad breath and a pale tongue