The sentence "あなたの背中にメロンパンが付いていますよ" actually means "There is a melon bread attached to your back." It is a humorous and somewhat nonsensical phrase that is sometimes used in Japanese entertainment media.
"I love you" in Japanese is typically expressed as "愛しています" (aishiteimasu) or "大好きです" (daisuki desu), among other phrases depending on the context and level of formality.
I’m professional 😊