The "Divine Language" spoken by Leeloo was invented by co-writer and director Luc Besson, and further refined by Milla Jovovich, who had little trouble learning and developing it, as she was already fluent in four languages. The language had only 400 words. He and Milla Jovovich held conversations and wrote letters to each other in the language as practice. By the end of filming, they were able to have full conversations in this language.
Luc Besson demanded that most of the action shots in the movie take place in broad daylight, as he was reportedly tired of the dark spaceship corridors and dimly lit planets common in science fiction movies, and wanted a brighter "cheerfully crazy" look as opposed to a gloomy, realistic one.
When composer Éric Serra showed soprano Inva Mula (who dubs the voice of the Diva) the sheet music for the Diva Dance, she reportedly smiled and relayed to him that some of the notes written were not humanly possible to achieve, because the human voice cannot change notes that fast. Hence, she performed the notes in isolation, one by one, as opposed to consecutively singing them all together, and they digitized the notes to fit the music. There are a few moments when you can hear the differences in the vocal tones of the Diva's voice.
Luc Besson wrote the original screenplay when he was in high school. He had conceived the story of this movie and invented the world of the movie as a child so he could escape his lonely childhood. He began writing the script when he was 16, though it was not released in theaters until he was 38.
At the time, it was the most expensive movie ever produced outside of Hollywood, most expensive production in Gaumont's history, and at $80 million USD, the visual effects budget of the movie was the highest of its time.