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At the beginning of the film, Shosanna doesn't speak or understand English, but the rest of the film takes place three to four years later which is more than enough time for her to learn English, possibly to avoid another situation like the beginning of the film. Also, it has been mentioned in many books that Adolf Hitler may have known a little bit of English (probably not French), so Shosanna might have wanted Hitler to understand the words she spoke (as shown when Hitler stands up and angrily shouts out "Enough! Stop (the film)!" after her film announces all the Nazis in the theater are going to die). Lastly, Shosanna's use of English is a simple continuity choice, as it is meant as a direct response to Frederick Zoller's line "Who wants to send a message to Germany?", which is spoken in English.The technical answer is that the scene was originally supposed to be in French, but Mélanie Laurent suggested to Quentin Tarantino that it be done in English. Most likely because it would be hard to read subtitles during all the chaos.
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