Japanese uses sound words far more than English does, and not just for noises. A large share of everyday speech leans on them, which is why a flat translation can drain the life out of a sentence.
Two families
The words split into two groups that English mostly lumps together.
Giongo: actual sounds
These imitate noises, the way "bang" or "splash" do in English.
wanwan, a dog barking
zaazaa, heavy rain pouring
Gitaigo: states and manner
These describe conditions, textures, and feelings that make no sound at all. English has almost nothing like them, which is where translation gets hard.
kirakira, sparkling or glittering
pekopeko, being very hungry
dokidoki, a pounding, nervous or excited heart
Why they resist translation
Take わくわく (wakuwaku). It is the bubbling feeling before something you are looking forward to. English makes you choose a single word, excited or thrilled, and you lose the specific flutter the Japanese carries. The reverse is worse: there is no tidy Japanese sound word for "awkward silence", while Japanese has しーん (shiin), the sound of silence itself, which is a sound that makes no sound.
Onaka ga pekopeko da. I'm starving.
Hoshi ga kirakira hikaru. The stars are twinkling.
Where they pile up
Manga and casual speech are saturated with them. A single panel might use a sound word for footsteps, another for a character's mood, and a third for the weather, none of which a literal engine handles well. This is also why machine output of manga or chat messages can read oddly: the engine renders the dictionary gloss of a sound word and the texture vanishes.
Reading them yourself
Two quick patterns help. Repeated kana, like ぴかぴか or もちもち, almost always signals one of these words. And a word ending in a small り, like こっそり (secretly) or ゆっくり (slowly), is usually describing manner. Spot the pattern and you can guess you are looking at a sound or state word even before you know its meaning.
Try a sentence in the translator and compare it with the examples above.
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