i think if you're worldbuilding psychiatry and mental health into your sci-fi/fantasy world then your first consideration should NOT be "what words would they use to describe our world's mental disorders??" even though that Can sometimes be useful.
granted, i DO prefer a world that slaps "melancholic" on in place of "depression" or "post-birth hyper-grief parental syndrome" onto postpartum depression or whatever, as opposed to just calling everything depression and schizophrenia and ASPD, as if our own western psychiatric words are the """correct""" ones and would logically be emulated by the whole of any other universe.
i think it is MORE useful to think FIRST about your fictional society's values. and the power structures in play, or lack thereof. and how elements like religion and spirituality and magic and sci-fi tech shape and influence everyday life.
mental disorders and neurodivergences are things that mark a person "other" from someone who is the """expected""" product of their environment. crucially, in our own western conceptions of mental health, there is a STORIED history of people being labeled Other because they did not cooperate with the oppressive social norms of the time. see: insane asylums, lobotomies, drapetomania. etc.
how does your fictional society treat people who are Other? is mental health treatment more focused on compliance and the erasure of symptoms, or on self-acceptance and accommodation? is there a taboo against being labeled Other? what is the history of self-advocacy and human rights for people in these groups?
you don't Have to go the pessimist route and write in, like, horrific atrocities as a backdrop for your story. especially if you are not a horror writer like me. LOL.
but i do think it's disingenuous not to think about these things on a structural level.
what would these people consider a mental health disorder / neurodivergence, and why? what thoughts or behaviors cause that to be labeled a mental health disorder?
how are neurodivergent people in this world supported or unsupported?
what are the consequences for having an untreated mental health issue in this world? are there reasons someone might hide their mental health issue, or is there a baseline of trust socially?
if there IS an automatic baseline of trust, what social norms enforce that trust? what are the expectations of conduct for both the mentally ill and those helping them? what happens if that trust is broken by a mental health professional or carer -- what recourse does the mentally ill person have?
are there any Pretty Shitty social norms that would be reflected or encouraged in the mental health system?
these basic questions are Much more interesting than just going on thesaurus.com for the DSM-5. and the answers will make your world feel significantly more grounded.