My favourite translation quirk, of course, is the infamous Splash:
While its English name suggests an association with water, its Japanese name, はねる haneru, can mean both splash and hop, but usually the latter. This explains why the move can be learned by Pokémon such as Spoink and Hoppip—species who have no association with water but are known for hopping or jumping—and why it is disabled by Gravity. Its somewhat misleading English name is due to its initial exclusivity to Magikarp.