Does anyone know how to translate or any method to understand the xfx and / or onomatopoeia of the one piece manga? If anyone knows please give me a hand.
I find The JADED Network to be a good English glossary for Japanese onomatopoeia and mimetic words. Normal Japanese-English dictionaries usually aren't enough to handle the diversity of manga sound effects.
If you don't find a word in there, try fiddling with long vowels or small "っ"s a bit. In this case, I've shortened ガキイン Claang to ガキン Clang, and found it.
It is sound ガキイン, four texts, one by one read as "ga,ki,i,n". It sound like means slash on target. Read it fast, you will know the result of sound given by these onomatopoeia. At left side, small ハア、ハア is "haa", is a whispering sound haa~ haa~, repiration sound, tired sound.