"Ñuhi bartos geltose jomīsan."

Translation: I am wearing a helmet on my head.

[purplepurpur]

Why does this go in the accusative instead of the locative?

[Xenon-]

It is the nature of the verb. "jomīsan" if translated very standardly translates to "I keep protected". So:

Ñuhi bartos geltose jomīsan. = I keep my head protected with a helmet. » I wear a helmet on my head.

It is that two different languages use different syntax to achieve the same meaning. It happens!