In most cities, the bootcamp is taught in English.
In French cities, the program is in French. You will have a 1h30 lecture in the morning in French, and a 1h30 live-code in the evening in French as well. So, if you don't understand French correctly, you won't be able to attend the bootcamp in France.
In some other cities (São Paulo, Shanghai, Chengdu, Tokyo) specific sessions are organised in other languages (Brazilian Portuguese, Chinese, Japanese). You can check the language of the next batch on the "Apply" page.
Extra info: all the challenges' instructions and written documentation are in English, so all students must have a good level and understanding or written English, even in cities where lectures are given in another language.