What is the word 'heart' when translated from English to Japanese?

心: Kokoro

(名) 心臓

The name of the heart, the organ, is 心臓 shinzou. However, 心 kokoro "heart, mind, spirit" or 胸 mune "bosom, breast, chest" are sometimes used to express the abstract idea of the "heart" as related to emotions.

And the love heart is 'har~toh'

Kokoro is more soul meaning.
心 /ko ko ro/ means 'heart' in sense of 'mind, spirit'.

心臓 /shin zou/ means 'heart' in sense of the body part. ハート /haa to/ is the same Word inserted into Japanese and can be used for both senses.