Is the rise in the western world of 'mental illness' such as 'depression' a reflection of language usage or some more substantive reason(s)?

As Sally Haslanger writes, proper answers to your question do need to feed on some empirical, in this case historical data. But I suggest that these data can only yield answers through some philosophical unpacking. The category of mental illness is itself a cultural product, insofar as it depends on the notion that 1) our minds can be ill just as our bodies can; 2) and so that they are instantiated, most probably, by the brain. Note, however, that to restate the concept of mental illness in such terms is not to say that it is merely a cultural product, that truths are relative to cultures, that if some people believe our mind is instantiated by our feet then so be it. I say this because the kernel of your question seems to address the matter of whether it is useful to imagine a clear-cut distinction between language use - that is, broadly, culture - and a "substantive" reality. To this I would answer No: what is described by the expression "mental illness" must correspond to a reality which...