For a long time, the classical languages Latin and Greek were major subjects in English schools. Mai Osawa, using Colin Shrosbee's Public Schools and Private Education (Manchester University Press, 1988) as a source, writes that "the French Revolution and the Napoleonic Wars reinforced the importance of national languages as an expression of patriotism." This led to the introduction of new subjects, including modern languages. Osawa writes,
From the end of the eighteenth century, boys started to learn modern languages like French and German as important subjects in the public or grammar schools. It is natural that the English characters in The Professor graduated from public schools or grammar schools can read, speak and write French.
Osawa also adds,
On the other hand, the French language in British girls' schools was more significant than in boys' schools.
Apparently, Charlotte Brontë loved the French language: at the age of 16, she wrote a letter in French to her friend Ellen Nussey. In 1842, Charlotte and Emily travelled to Brussels, where they could improve their French while working as teachers. In Charlotte's third novel Villette, the main character travels to the fictional French-speaking city of Villette to teach at a girls' school; this idea was based on the author's own experience.
For this reason, it is not surprising that we find French in all of Charlotte Bronẗ́ë's novels, especially in Villette. According to the blog Charlotte Brontë’s Love Of The French Language,
Her publisher George Smith begged her to allow the use of translated passages after her French ones, but Charlotte stood firm and insisted on French only in the passages she had so written.
Rightly or wrongly, it seems that Charlotte Brontë expected her readers to know French. Her publisher apparently disagreed.