① The letter was written in French, which language I did not understand.

= The letter was written in French, and I did not understand that language.

② She favours unisex, which idea I’m quite opposed to.

= She favours unisex, but I’m quite opposed to that idea.

③ He had completely misjudged the situation, which fact he later discovered.

= He had completely misjudged the situation, and he later discovered that fact.

④ He kept to his bed for a week, by which time he had recovered.

= He kept to his bed for a week, and by that time he had recovered.

⑤ The ship ran short of provisions, which circumstances obliged her to put in at a port.

= The ship ran short of provisions, and that circumstances obliged her to put in at a port.

⑥ The first sense of sorrow I ever knew was upon the death of my father, at which time I was not quite five years of age.

⑦ He stayed there for a month, during which time he went through a series of hardships.

⑧ Possibly they're all lying, in which case it's hopeless and we might as well give up.

⑨ I called him by the wrong name, for which mistake I apologize.