If everybody in the world woke up suddenly speaking one language, leaving all others forgotten, I'd pick Kabardian as the sole surviving tongue of this paranormal linguistic mass extinction. Specifically Kabardian written in the Mifo-Circassian script. You wouldn't have to worry about accessibility since everybody just magically knows the language, so why not pick my favorite language that just so happens to be very hard to learn?

Another route I could go would be to invent a language combining my favorite sounds, letters, and features in different languages, to preserve as much linguistic diversity in a single tongue as possible.

But frankly, I don't think I want either of these to happen. Languages are awesome, and as great as it would be if everybody could understand each other, getting rid of every other language would be more than a tragedy.

Not to mention, if everybody woke up speaking one language, I'd imagine there'd be a period of widespread chaos as people suddenly lose the ability to understand traffic signs and user interfaces across the majority of the world.