Khmer

Khmer has 33 base consonants (each with a 'first-series' and 'second-series' form determining the inherent vowel), and around 23 to 24 vowel characters that combine with consonants. Adding subscript consonants (used for clusters), independent vowels, and various symbols, the practical character set runs to about 74. The system is comprehensive enough to spell virtually all Khmer phonological distinctions but takes substantial time to learn.

No — Khmer is non-tonal, unlike its neighbour Thai and its distant relative Vietnamese. Khmer compensates with a rich vowel system: the language has roughly 25 vowel sounds (long, short, diphthong) and uses vowel quality and length to distinguish words rather than pitch. This makes Khmer relatively unusual for the region — most major Southeast Asian languages are tonal.

Cambodia is the heartland, with around 16 million native speakers. Substantial communities live in southern Vietnam (the Khmer Krom population of the Mekong Delta), eastern Thailand (the Khmer Surin), and across the Khmer diaspora in the United States (especially Long Beach and Lowell), France, Australia, and Canada — many tied to refugee migration from the Khmer Rouge era of the 1970s.

Yes — both Austroasiatic, but in different branches. Khmer is the largest Mon-Khmer language; Vietnamese is in the Vietic branch. The two diverged thousands of years ago and modern speakers can't follow each other. Khmer is more closely related to Mon and several smaller Austroasiatic languages of mainland Southeast Asia. Vietnamese has been more heavily influenced by Chinese contact than Khmer has.

The script is the steepest part of the curve — substantial character set, complex consonant clusters, and orthographic irregularity from preserved historical spellings. The grammar is light: no verb conjugation, no noun declension, SVO order. The non-tonal phonology makes pronunciation more accessible than Thai or Vietnamese for English speakers, though the rich vowel inventory still takes practice.