Answer:
In Paris but not in France, the thinnest of its siblings.
I'm white, I'm round, but not always around. Sometimes you see me, sometimes you don't.
The higher I climb, the hotter I engage. I cannot escape from my crystal cage.
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
Dies half its life. Lives the rest. Dances without music. Breathes without breath.
Be sure to shout for its answers are weak, but there is no language it cannot speak.