The two
Aku cinta kamu. ‘I love you.’
Aku sayang kamu. ‘I love you.’
Both translate the same way and they are not interchangeable.
Cinta is romantic love, and it is heavy. It belongs to declarations, to song lyrics, to the moment things become serious. Said too early or too casually it sounds theatrical.
Sayang is affection. It covers love for a partner, for a child, for a parent, for a close friend. It is warm rather than dramatic, and it is what people actually say to each other day to day.
If you are unsure which one the situation calls for, it is sayang.
Sayang as a name
Sayang is also the word you call someone, the way English uses “darling” or “love”.
Sayang, sudah makan? ‘Have you eaten, love?’
You will hear it constantly between couples and from parents to children.
The trap
Sayang has a second meaning with no connection to the first: what a shame.
Sayang sekali. ‘What a pity.’
Sayang, hujan. ‘Shame, it’s raining.’
Same word, same spelling, and only the situation separates them. Nobody will misunderstand you in practice, but the first time you hear someone say sayang about the weather it is disorienting.
The step below
Aku suka kamu. ‘I like you.’
Suka is like, for people and for things. It is genuinely a smaller statement, and it is the safe one early on.
Missing someone
Aku rindu kamu. ‘I miss you.’
Aku kangen kamu. Same thing, and this is the one people say.
Kangen is Javanese in origin and has spread nationally. Rindu is the literary form.
What you’ll actually hear
What the book says: Saya mencintai kamu.
What you’ll hear: Sayang kamu. or, in a message, just ILY.
The formal mencintai with its me- prefix belongs to writing. Nobody says it out loud to a person they are in the room with.