If "fluently" describes speaking or writing:
* Easily: She spoke English easily.
* Effortlessly: He wrote the essay effortlessly.
* Articulately: They presented their ideas articulately.
* Smoothly: The conversation flowed smoothly.
* Gracefully: She handled the difficult question gracefully.
If "fluently" describes something other than language:
* Effortlessly: The machine operates effortlessly.
* Smoothly: The transition between scenes was smooth.
* Seamlessly: The project was completed seamlessly.
* Naturally: The dance flowed naturally.
Other options:
* Without hesitation: He answered the question without hesitation.
* With ease: She navigated the complex system with ease.
The best adverb to use will depend on the specific situation and the desired nuance.