General antonyms:
* Energy
* Vigor
* Vitality
* Excitement
* Liveliness
* Alertness
* Activity
More specific antonyms:
* Exuberance (for a feeling of high spirits)
* Enthusiasm (for a feeling of strong interest)
* Alertness (for a state of mental awareness)
* Strength (for physical power)
The best antonym will depend on the specific meaning of "languor" in the context you are using it.