languor vs lethargy

languor and lethargy both are nouns.

Word	Noun	Adjective	Verb	Adverb
languor	Yes	No	No	No
lethargy	Yes	No	No	No

As nouns, languor and lethargy are synonyms defined as:

Other synonyms of languor include flatness, phlegm, sluggishness.

languor (noun)	lethargy (noun)
inactivity; showing an unusual lack of energy	weakness characterized by a lack of vitality or energy
a relaxed comfortable feeling	inactivity; showing an unusual lack of energy
a feeling of lack of interest or energy	a state of comatose torpor (as found in sleeping sickness)
oppressively still air