薄荷不懂柠檬心薄荷不懂柠檬心
[bó hé bù dŏng níng méng xīn bó hé bù dŏng níng méng xīn]
This translates literally as 'Mint doesn't understand the heart of lemon.' It may suggest that someone is too simple, straightforward or naive (represented by mint) so he/she finds it hard to fathom complicated or sour emotions (suggested by the lemon heart).