It seems the stain is already on the wood - both inside and out.
And it seems the raised bed will be used to grow food.
The question raised is, "What to line the inside with?"
I'd use a stiff plastic sheeting. Bamboo shield would be perfect. It comes in varying widths and thickness. Bury the shield vertically and flush against the inside wooden walls so the top of the shield is level with the top of the wood, to make a wall between the soil and wood.
the plastic is not toxic to the soil or plants and will last forever (which is not necessarily a good thing.)
