They are different concepts. Torpor essentially just means "slow and lazy", scientifically it refers to a down regulation of body temperature and/or metabolism, but doesn't reflect on the REASON for it.

Lethargy is slowness specifically caused by how you feel - fatigue, nausea, dizziness, sleepiness, that sort of thing.

Languid is a closer synonym to torpor, although it can actually refer to either meaning, essentially (slow OR tired).