"Sweetheart" is most often rendered as "Chérie" (fem.) and "chéri" (masc.) in French.
In Cajun French, you can say "cher(e)" or "mon cher" to mean sweetheart.
Mon chéri.
Merci chérie
ma petit chou
Là où êtes vous amoureux
ce qu' une petit amie
Bonne nuit mon Cher
tu me manques mon cœur
bonjour chérie
Passe une bonne nuit, chéri(e).
"Chérie" is in French; it means "sweetheart" or "dear." "Tu es ma chérie" means you are my sweetheart (if you are saying this to a girl). For a guy, you would say "Tu es mon chéri."
Cherie