Earth

His explanations are adequate but I think he's confounding a few linguistics concepts that make it sound more complicated than it should

Sperm whales appear to have vowel length, similar to how in english 'bit' and 'beat' are distinguished by the length of the i short and i long vowel. Mandarin Chinese doesn't possess this quality. Vowel length does not distinguish between words in Mandarin

Sperm whales appear to have tonality, which is the pitch at which vowels are produced, or the movement of the pitch during the vowel. Think of the difference between a strong declarative No! \ with a falling pitch, or a questioning No? / with a rising pitch. In English, pitch is used at the word level for emphasis. In Mandarin it's at the vowel level to distinguish between words. Both languages posses tonal qualities, but employ them to different ends

Sperm whales appear to have diphthongs, which are two resonant vowels produced in sequence, sometimes with a 'gliding' effect. A diphthong is like the 'iiieee' sound of 'pie' (the I and E vowels are created with resonance produced with different shapes in the mouth, and there is no interventing full or partial closure of the vocal tract (consonant) so it's called a diphthong). Both English and Mandarin have diphthongs.

So Chinese being the 'hardest' language is a furphy. It's just less related to English than Italian is to English, and some of its linguistic qualities are employed differently.