They know all the key words a dog needs to know: those for walks, car rides, food, toys, and anything else that drives them to fits of doggie bliss, but they also know some more obscure ones, like "garage," "upstairs and downstairs," the names for various rooms in the house, the names of all their much-loved people and stuffed toys and, inexplicably, in one case, Elvis. And most unfortunately, they know the names for things we wish they didn't.
And it's these words that denote things, behaviors, or people that we try to hide from the dogs that sometimes makes talking around them difficult. Say something enough times and eventually a very smart dog will learn to associate the word or phrase with the action that accompanies it. And so we began to use Pig Latin when speaking of those forbidden things. That is, until it became apparent that the boys had developed an understanding of a fair amount of Pig Latin, which then forced us to speak in French. And now they're starting to comprehend the French. We figure that we have a bit of time left before we've exhausted all the more common languages on earth, which buys us some time before we're reduced to speaking of forbidden doggie things to one another in a remote African language that consists entirely of clicks and whistles. But at the rate we're going, it may be sooner than we think.