Hello Hela,
1) Could I describe a crusty old woman as being surly, churlish, bad-tempered?
— to my mind, those adjectives might coincide with "sullenness", but not necessarily with "crustiness". However, other members might disagree!
2) nonplussed = puzzled, perplexed ?
— that's it!
3) Should I understand:
a) "talk at" in the text a having a one-way conversation?
— I think so, yes.
b) "talk past" and "talk through" as talking without being listened to or without touching / interesting the other?
— "talk past"/"talk through" suggest to me that the talker isn't taking any serious notice of the person who is "talked past/through".
4) What should I say here:
a) Their private life/lives ?
— "life" e.g. when speaking of a married couple; "lives" when speaking of unconnected people.
b) They are not concerned with each other’s life/lives.
— I would choose the plural.
5) How would you translate "selon lequel / laquelle" in English?
ex: un principe selon lequel une personne ne peut....
— hmm; that might depend on the full sentence! Can you give me more context?
6) I found these sentences on the net could please tell me if they are written in correct English?
a) To let on to people about a secret.
— yes; but probably more often found in "informal" English.
b) To stress a heavy emphasis upon an issue.
— no; "to put/place" a heavy emphasis upon an issue". The emphasis is not itself "stressed"; the emphasis is the stress!
c) The British are finicky about maintaining social rules and usually think it is undone to shatter the social code of behaviour.
— no; it should be "not done", which means "not acceptable". So the clause is something of a tautology...
"Undone" relates to buttons, maidens, and shoelaces.
All the best,
MrP