They come across multiple planets, with one transmitting very obvious radio signals. It looks habitable, lacking any huge landmasses dominating most of the surfaces.

Planets?!  

The fleet makes sure they aren't about to run into an asteroid or something (still unlikely within a star system, but less unlikely than in interstellar space, and worth checking for even out there after random teleportation).  There's a quick scan of every critical system and a check that none of the inhabited halls are losing air.  One of the anti-gravity bulbs blew out, though that happens from time to time anyway and might not be related.  Otherwise, the ships really do seem fine.

Concurrently, external communications are aimed towards the planet to try to piece together the signals.  They've had generations to consider just what to do in the event of discovering aliens, but had assumed they'd either be meeting someone in space with them or have time to talk while approaching the star system.  Still, even as their own computers start churning to try to figure out the signals they're picking up, they begin to send out some of their own.  Clear, intended contact, starting with basic math.

People invented and speculated on ways of meeting and communicating to aliens for fun, so the rare Alien Contact Clubs get quite shocked and surprised at seeing unmistakably intelligent signals at wildly unexpectedly strong amplitudes!! A mishmash of symbols meant to be maximally intelligible get returned, built with the care and attention of a small group of dedicated hobbyists.

The Fleet's Alien Communication Procedures are under the jurisdiction of the Captains rather than individual hobbyists, and thus a bit flat and official.  On the other hand, the people who choose to work at them are ones who found the idea of possible alien contact to be more interesting than any other job.  They reply as best they can, though stick with relatively basic patterns until they're absolutely confident they have enough material to try something more advanced.  No one wants to accidentally declare war.

Navigation, meanwhile, has been calculating the paths of the planets in the star system.  They won't make moves to change their current heading until they've received permission to approach, but they should probably figure out how long it would take to reach orbit. 

They get multiple replies in fact! It's messy and it's tough to get a definitive reply, but they don't hear many people saying no to them approach, outside of those worrying about illnesses.

The ships turn their thrusters and begin aiming towards what will be a high orbit around the inhabited world.  At the speeds they're capable of, it will still take a bit over a month, which should be plenty of time to iron out any difficulties and make plans for a first contact with all appropriate quarantine and biological safety requirements.  They don't have everything ready-made, but as the news of where they are spreads throughout the internal halls of the ships people set to work preparing for anything they might need.

The fleet has all sorts of questions.  Most pressing of which is: How did we get here?  

They explain their position - The Fleet is a set of generation ships that were sent out as a last shot of survival when their own world was on the brink of destruction.  They had been in deep space, still around 70 years out from the nearest likely-inhabitable planet.  They're pretty sure it wasn't this one - the star is the same type but the prospective planet was a bit smaller and had far less water and the other planets in the system don't match either.  

Generation ships cool... says a few of the radio replies, that slowly die out as many voices turn to one, to make it easier for the aliens. One month is very sufficient for a safe and nice quarantine, there are depopulated and abandoned farming villages in all climates and all places ready for new inhabitants to move back to them, they need only know how many people they need to prepare for and where they'd most want to be. 

Trying to get some file formats that work, the planet transmits signals of helpful dictionaries with images to explain the languages!

The brightest minds among the prospective diplomats are tasked to learn as much as they can of what seems to be the most common language.  Their own dictionary is shared, in turn.  Generations aboard a ship with one language has left their own translation systems somewhat lacking, but they have plenty of manpower for updating it.

"There are just shy of 1 million passengers.  Our internal ship climates are subtropical but we can live in a wide range outside of that.  The ships' halls were created with the intent that they could be landed safely and turned into arcologies, however the preparations will take some time."

(In truth, less time than is implied.  But there is some suspicion regarding just how they got here and just how very warm the welcome is.  The captains had convened and decided that the information would be important to mention, in case everything really was as good as suggested, but a delay might keep them better able to escape if needed.)

"How did so many villages become depopulated?"

There is brilliantly one language that manages to be almost totally ubiquitous, with a written, spoken and signed form. The dictionary sent in return gets practiced by communities of curious enthusiasts.

"We can similarly handle and provide a wide range of habitats, though likely not for 1 million subtropical residents in quarantine."

"Depopulation happens due to urbanization of the agricultural workforce, or the exhaustion of natural resources."

"Having a single ubiquitous language is convenient!  The histories of our original home world implied they'd never managed to come to a consensus, though our ancestors who left on this particular fleet were all fluent in Kantoan.

"The existence of an entire language's worth of hand signs is a new concept to us.  We have several gestures and body language signals, such as nodding to show 'yes', but hadn't considered them part of language in the same way that speech and text were.  It seems like a good thing to have."  A collection of common fleet body language is compiled and sent along, including sets for both their human halves and various daemon body configurations.  The Fleet will be doing their best to mimic their hosts, given that they're on their territory, but knowing ahead of time of any conflicts can help to explain any accidental slip-ups if someone forgets later.

Conveniently enough, the sign language ad the boy languages end up different enough to stop misunderstandings for being a plausible issue. They're quite shocked to learn about the daemon creatures! It leads them to ask what they wish to trade with them, once they've landed.