I have heard you are "sick in the head in English" enough that it seems perfectly acceptable to say about mental illness. I think saying "you are sick" would have more of a connotation of very abhorrent behaviors than mental illness.
I wouldn’t consider it an acceptable way to refer to mental illness. Someone I would refer to as “sick in the head” actually wouldn’t be mentally ill but an evil, manipulating, scheming [bleep]. The kind of person who takes perverse pleasure in causing others misery. That is someone who is sick in the head.
Which makes it a good translation for the Danish sentence, though. :)
"Syg i hovedet" is more about being a psychopath/sociopath than about.. um, other mental illnesses.
Would it not be acceptable to say 'You are mentally ill'? This would be a fact in England. But 'You are insane' would be more like an insult.
No. When Danes say, "Du er syg i hovedet", it is normally meant in an insulting way, either friendly or hostile. The translation of, "You are mentally ill", would be, "Du er psykisk syge", amongst others.
English isnt Danish. The different phrases arent expected ro gave a 1:1 correlation