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According to my web search, Emilio Cossira played Juliet in the 1900 film version.
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I think Emilio actually played Romeo, and although it was not unheard of for the French to act "cross gender" (consider that Sarah Bernhardt played Hamlet in the fencing scene of the silent film released the same year (June 8, 1900) by the same group of film-makers, the Phono-Cinéma-Théâtre, for the World's fair in Paris). There doesn't seem to be any such indication that Emilio played both Romeo and Juliet.
I believe the truth is that it was some unknown contemporary of Bernhardt, and that "Romeo-Juliet" was among the 24 films that toured Europe following the 1900 World's fair until 1902 and then were lost until the 1930s. A number of the films were rephotographed in 1952 but I don't think Romeo-Juliet was among them. A true scholar of early film might be able to give better information, but I'm guessing that this question is nearly impossible to answer from internet sources.
There's no reason to suppose that there was a "Juliet". This was not the Shakespeare play. Cossira was a tenor from the Orvieto opera house and was simply singing an aria from Charles Gounod's 1867 opera Roméo et Juliette. An ad for the Phono-Cinéma-Théàtre on tour in Germany (to be found on the web) specifies that it was the Gounod opera. I am a true scholar of early film but this information all comes from internet sources