Modern Greek.
I’m going to list indeclinable, straight iconic words, as opposed to the far larger set of inflected words with an onomatopoeic etymology (like zuzuni for bug or platsurizo for to splash).
- bam bang, dan ding dong, apsu < Turkish hapşu sound of sneezing, kix cough, xrats scratch, drin ring ring, ksu shoo, prits blowing raspberry, ɣlu glug glug, sut shoosh, psit psst, ftu spit
- ɣav bark bark, njau meow, tsiu cheep cheep, kokoko cluck cluck, kokoriko cock-a-doodle-doo
Modern Greek also has a quite elaborate baby talk register partly based on onomatopoea. (In fact, Tsakonian has its own distinct set of baby talk words!)
- mam eat, dziz fire/danger, pipi pee, kaka poo, nani sleep, tsa peek-a-boo, nini baby, ata < Italian strata walkies, tutu car