Lingua Franca
A year ago.
What were the chances that the most salient language of the humans five decades ago could still be considered of sufficient relevance now? Valcen considered the constraints: By one measure it was one and a half of their generations, enough for a sharp memetic drift given dire circumstances that he had no reason to assume had occurred, though the probabilities of strife were high enough that he couldn't rule out a significant shift of power. Regardless, even in the worst case, the knowledge of their global trade language from half a century ago would not have been erased - but whoever was sent their way would be young by their standards. Even if they sent a linguist, there was no reason for that individual to be equipped with awareness of that tongue in particular; they would be sent to speak to a culture without any knowledge of Earth, after all, there was ostensibly no reason to bring along many Earth languages.
But if they brought knowledge of that language with them, Valcen had to speak to them directly. There was no teaching this language to the Nayabaru. They were perfectly capable of learning it, of course, but Valcen had a poor grasp of its grammar, basic though it seemed on the surface. Ultimately, it was one thing to speak it and quite another to teach, and he had never thought to remember enough of it to be certain in his analysis now. He was fortunate he had thought to pack some of it at all.
Humans. In their own biological classification system, Homo sapiens sapiens. Physically trivial to overcome, mentally advanced. Comparable to the Nayabaru in their desire for social structure. Not comparable to the Nayabaru in that they were blessed with a staggeringly refined ability to lie - both to others and to themselves. A predator species, naturally curious. A creature that had elevated 'playing' to an art form. A creature that had explored its own mental diseases more thoroughly than any other species Valcen had ever known - and embraced half of those it had found, although it depended on the kind of culture one observed as to which of the flaws made it into the pot.
Depression, schizophrenia, anxiety. Authoritarianism, sadism, disloyalty. Nationalism, paranoia, narcissism. Cruelty and cohesion, or permissive, gradual dissolution.
It was a broad set of behaviours and attributes to choose from. Daunting, perhaps, to one who had not spent thirty years meticulously exploring the common underpinnings, observing his own neurons and the biochemistry surrounding them from the outside more literally than most.
In isolation, they were simple to handle. If they had no other human to mirror, their resolves quickly faltered, even absent pressure to do so. Properly functioning human beings - and if the world had not completely changed in his absence, only properly functioning human beings would ultimately be sent their way - avoided conflict and sought to please, especially in unknown situations appearing like avenues to peace. Their empathy was easy to play. Their loyalties on the other hand were unfortunately difficult to break without an application of force that would have other even less desirable side-effects. Whoever got to them first would imprint them.
Plan A was likely the only viable approach. Plan A meant avoiding making an enemy of them at all costs. Plan A was a controlled first contact.
Watch the skies. Attempt to contact any vessel you think may come from Earth with a friendly hail. Use these exact syllables: "Hello - we are the Nayabaru. We welcome you to our world."