In what order to apply lighting and translucency?
When blending multiple layers, physically the "right" thing to do is calculating lighting on each layer separately, then composite the lit layers together. This way, for instance, you can have a translucent sprite standing in a spotlight, in front of a dark background, and the sprite will be well-lit while the background visible through it stays dark. Or conversely, you can have a dark sprite silhouetted against a brightly lit background.

If you blended the sprite with the background before lighting, it wouldn't be clear which lighting environment you should use if the sprite and background have different ones.