Joseph Conrad once wrote: 'who knows what true loneliness is, not the conventional word, but the naked terror. To the lonely themselves, that wears a mask, the most miserable outcast hugs some memory, or some illusion
There's a mistake in the quotation, which explains why the English text doesn't make a lot of sense, and why it is next to impossible to translate it into French correctly. The key word is "HAPPINESS", which should be replaced by "LONELINESS". See now:
Who knows what true loneliness is – not the conventional word but the naked terror? To the lonely themselves it wears a mask. The most miserable outcast hugs some memory or some illusion. – Joseph Conrad http://kenfran.tripod.com/quotlist.htm#Conrad
Now it makes all the sense in the world and can be translated into French without any problem.