Although "glitter" and "slippery" may sound onomatopoeic, neither one is actually onomatopoeia.
The latter case is perhaps the easiest, since "slippery" is an adjective derived from the verb "slip", still in use. (The word "glitter" is also derived from a verb, but this verb is no longer used.) Generally, words that are onomatopoeia don't have an etymology like that; if they did, then the sound would not be the result of trying to create the impression of something. For example, the "-y" in "slippery" is there because it's an adjective. The extent of onomatopoeia in Japanese is indeed quite different from English (and Chinese).
The latter case is perhaps the easiest, since "slippery" is an adjective derived from the verb "slip", still in use. (The word "glitter" is also derived from a verb, but this verb is no longer used.) Generally, words that are onomatopoeia don't have an etymology like that; if they did, then the sound would not be the result of trying to create the impression of something. For example, the "-y" in "slippery" is there because it's an adjective. The extent of onomatopoeia in Japanese is indeed quite different from English (and Chinese).