Golden retrievers can have partially black tongues. While the Chow Chows and Shar-Peis are commonly-known breeds with fully black tongues, many other purebred dogs, or dogs with mixed heritage, can have black spots or sections on their tongues.
One should not assume that a Golden Retriever is not purebred because he or she has a partially black tongue; this assumption is commonly made by shelter personnel or those who are not knowledgeable about this breed characteristic.
The Chow Chow is one of the oldest breeds of dogs, so many dogs may very well have the trait in their line of descent. Seek professional advice to confirm the breed of a dog.