Gods in this world are not physical beings, but instead a consciousness that can span a great distance. They think many times faster than a human, but lack the ability to multitask--so they can observe a whole continent with a coarse grain knowledge of what is happening, or "zoom in" on specific cities or even individuals to "see" much more detail, but not both at once. This can happen very quickly, but due to the lack of control over what they can see at any one time (and they absolutely cannot be observing two different points at the same time), gods usually choose to delegate their will to mortals.
From comments: Gods usually choose to interact directly with humans via projections and don't personally have issues with language, but as this suffers from their inability to handle multiple tasks at once they can't really manage the translation themselves or teach a language to their chosen.
How can such a god ensure that all humans in a finite area speak the same language? It doesn't want to have to wait for two mortals empowered to enact its will to stumble through learning to communicate with one another even if they were born and raised on opposite ends of its domain.
Euphemisms, dialects, and accents are free to change. The god only cares that they can understand one another.
The god would like to avoid any modifications to the humans it talks to, because bad things can happen if it tries to modify someone too much (a different plot issue entirely), but it is willing to do very minor modifications if necessary to your answer.