The tags for speech and utterances can already be manipulated with syndromes, which means they can vary across individuals. That means there's definitely no need for a whole new caste.
Well alright, that's something I hadn't known. I just took a shot in the dark with the caste option not knowing how strong syndromes can be. If they can be manipulated with syndromes, then that's definitely a better way to go on it.
As an aside, does that mean it's outright possible to make a poison that destroys the victim's ability to speak while it's active?
Treat it like a secret, with a in-built (unless they have a poor grasp of speech and emotion themselves) parents desire to teach their children (or convince someone else to) this and bam hey presto a functional member of society that can interact positively with other dwarves, make friendships and maybe even fall in love.
> Dwarven Naturist (/critical thinker/other?) Urist Aarticulate apprentices the adopted kobold Urist Kneelneesi
> Dwarven Naturist Urist Aarticulate divulges the secret of 'dwarven language!' to the adopted kobold apprentice Urist Kneelneesi
As an aside entirely, what about a flat option to teach language to utterances creatures through the animal menu? If they're captured, then the ability to train them in some way becomes possible. Seeing as animal trainers would be used to dealing with creatures of an innate hostility towards them, then their experience could be useful to calming them and uh, ahem, "opening their minds" to the possibility of learning dwarven language and values that would integrate them into society.
Or perhaps just flat out conversion. Capture some elves, keep them in a uh... "culture school" which discusses the finer points of the practical usage of nature as well as apprenticeships in the logging industry. Or maybe just flat out slave labor their ability to create wooden objects through means non-harmful to the tree and then sell them to their kin without them knowing its source.