Khmer is the language spoken by Khmer people in Cambodia, parts of Thailand and elsewhere in the world where Khmer have emigrated.
Khmer is a complex script that requires special processing to display (like Indic scripts, Arabic and others).
Khmer does not place a space character between words (spaces are instead used somewhat analogously to commas in Western languages), requiring characters such as ZWSP U+200B (the Zero-Width Space) and ZWNJ U+200C (Zero-Width Non-Joiner) to be included in text for proper line-breaks by most applications.
What is problematic for Khmer inclusion is the fact that letters and diacritics can be stacked on top of or below the standard-height characters. So usually fonts created by Western designers shrink Khmer characters so that the maximum of 5 stacked glyphs is set at the same height as ascender-plus x-height-plus descender in the Latin glyphs. In contrast to Latin alphabets that have only a few characters with discriminating detail within the x-height span (a, e, k, s, x and z), Khmer contains a large number of glyphs distinguishable only by "dot"-size differences.