Let's begin. Humanity reaches for the solar system, but wants to make sure everyone can talk to each other. A "Solar Language" edict is enacted, making one language the default language for everyone not on Earth (And, eventually, on Earth too).
All goes well. People can talk to each other, as intended. Someone decides to create a generation colony ship to, say, Trappist-1. This colony launches, but it'll take a while to get there. This colony will be referred to as Colony 1, and it will be assumed that their language doesn't change significantly, specifically for the purposes of this question.
Meanwhile, some other idiot creates a bio-plague that almost exterminates the rest of Humanity. Humanity survives, loses pretty much everything, and rebuilds, but over the course of a few centuries. Of course, language changes. This set of humanity discovers a type of FTL drive, and plans another colony. FTL-Seeking Monsters attack and destroy Earth. Three groups splinter off:
- Colony 2, who escape on a ship with cryogenic suspension, but are awakened and form a colony with most of the tech from Dead-Earth.
- Colony 3, formed from ships that immediately flee but lack resources to establish technology. After a few centuries, they get back on their feet.
- Nomad Fleet, people who are picked up by the Friendly Aliens that follow the FTL-Seeking Monsters and save whoever they can. They go on their Friendly Alien way for the next few centuries.
So, a few centuries down the road, everyone comes together. We have Colony 1, who although they are the oldest colony, haven't changed their language. Their language is the original, which all others branched off from. Colony 2 has the least divergent language from Colony 1, and theirs is the basis for the language of Colony 3 and the Nomad Fleet. Colony 3 has diverged a lot from Colony 1, and a bit from Colony 2. They have the same base as the Nomad Fleet, but it's been a while. The Nomad Fleet's human population has the base language from Colony 2, derived from Colony 1, but only the same base as Colony 3.
How difficult would translation be between these groups? Would Colony 1 even recognize the language of the Nomad Fleet? Would Colony 2 be able to go "Hey, guys, I got this" and whip up a Rosetta stone for everyone? Would the Nomad Fleet and Colony 3 be able to talk to each other without much difficulty because they have the same base, or would isolation cause them to diverge too much?