Fire I, Fire II, Fire III

[GMERC]

Dunno how something as simple as numbers could bite them in the rear. What would they call the inevitable Fire V and potential VI?

This is actually an extremely poorly thought out post on your part. It doesn't matter what the localization has to come up with to call a hypothetical tier 5 spell because the Japanese version of the game will literally be using one either way.

The English localization is the only one they thought would be too stupid to understand the correct names. There isn't an actual good reason, it was just a stupid decision they're too prideful to walk back.

Fun fact: Stoneskin II is actually Stoneraskin (Stone II Skin).