Are there any languages which don't have Diphthongs at all?

Question

For example my native language, Georgian has no Diphthongs, Adjacent vowels do occur but they are heterosyllabic (source)

are there any other languages which don't have any Diphthongs and adjacent vowels remain heterosyllabic?

Answer 1

Yes, in fact this is generally the case in Bantu languages (there are hundreds) that vowel-vowel sequences other than same-vowel (i.e. long vowel) are two syllables. Likewise Tigrinya, Amharic and Tigre, as well as Arabic varieties (there are vowel+glide sequences which may be treated by some as "diphthongs"). This raises the question of what distinguishes diphthongs from vowel-glide sequences. The standard answer is that a diphthong in VV and a vowel+glide is VC, which implies having a diagnostic for C versus V. North Saami has both diphthongs and vowel-glide sequences, which act very differently in the phonology.

Answer 2

Yes, most dialects of Yi under the Sino-Tibetan languages don't have diphthongs. Like Chinese, Yi characters are monosyllabic. Adjacent vowels, which appear when the second character in a word doesn't have an onset, remain heterosyllabic.

The Yi language is quite interesting in that its rhyme is very simple, where a glide or a coda, either with a vowel or a constant, is impossible. Phonologically, all syllables in the language are open and only has one short vowel monophone (including syllabic m and m̥, ɿ, ʅ, which are also generally considered as "vowels" in the Sino-Tibetan phonology).

There does exist, however, several dialects of Yi which have a vocalic (thus diphthongs) or a constant coda, notably some in Yunnan have vocalic or nasal coda, while Bisu Yi in Thailand could have unreleased -p, -t, -k coda.