Summary:
potion.potency.NUMBER translation key (which show the strength of the effect in roman numerals) is not used in the effect list in the inventory, but is used in the potion tooltip (splash potions, lingering potions and tipped arrows).

Steps to reproduce:

1. Download and activate the resource pack bug_reproduction.zip.
2. Take a potion with a level from 1 to 6.
3. Look at the display of the strength of the effect in the tooltip.
4. Use any level potion from 1 to 6 on yourself.
5. Look at the display of the level of the potion in the inventory.

Observed results:
potion.potency.NUMBER translation key is not used in the effect list in the inventory.

Expected results:
potion.potency.NUMBER translation key will be used in the list of potions in the inventory.

Image: