It is not required to be liscened or certified in any state in US to train dogs.

You need to be liscened and certified as stated earlier if you are training Police units, DEA units and/or Service dogs for the blind.

So the answer to the original question, as to whether trainers are required by law to be certified train a dog is No. 99.9% of trainers certified or otherwise will never even get near a Police dog, DEA dog (unless they travel and bring back things they shouldn't) and most dog trainers I know are not blind.

As to the original question if there is a Blanket organization that oversees or regulates dog trainers as a whole. the answer sadly is No.

edit: the certification in DEA is simply so you may legally have in your possesion both natural and synthetic drugs in quanties for training.