Once we see the benefit of being able to communicate freely with anybody, and losing the protective aspect of being able to communicate in a language others can't understand, I think people will lose interest in local languages and dialect?
Why learn them when you can just translate them when you need them? What do they bring, except some aesthetical pleasure? Given the work it is to translate materials to a language, it will stop to be done: let the automatic system do that for you!
Then, people will be able to mix and travel more, exchange more, and so they will.
In this env, and using translation again and again, people will start to pick up whatever language is the most common.
Eventually, what is not useful will diseapear, only artefacts that we can translate automatically, will remain.