Resolved questions
how do you say "darling" in japanese?

darling = "ダ一リン".

Japanese use katakana to pronounce the words imported from foreign languages.

CASE 1 My darling
to boyfriend : (firstname)くん～ or nickname in a deux
to girlfriend : almost say firstname or nickname in a deux
to husband : あなた～
to wife : おまえ～ or firstname

CASE 2 Friend's darling
to boyfriend : かれし
to girlfriend : かのじょ
to husband : だんなさん or だんな or ごしゅじん
to wife : おくさん or おくさま