Performing a charge attack with a spear in your off hand applies the effects from the enchantments present on the item held in the main hand

When using the “lower spear” attack while the spear is in your offhand, any damage dealt by the spear will use all effects of the enchantments of the weapon in your main hand. (Example: Having a sword with “Fire Aspect II” in your main hand will ignite anything hit by your spear in your offhand)