The story "The Last Lesson" portrays the sudden realisation of the people in a small French town about the importance of their language. The following points demonstrate this:

Realisation of the Importance of their Language
The people of the town suddenly realise how precious their language is to them when they come to know that it is their last French lesson. The announcement of the Prussian rule, where only German will be taught in schools, makes them understand that their language is not only a medium of communication but also a symbol of their identity and culture.

Emotional Attachment to their Language
The students in the class feel emotional when they hear their language for the last time. They realise that they will never be able to speak their language freely again, and it makes them feel nostalgic. The teacher, too, feels emotional when he sees the students' love for their language.

Regret for not Learning Enough
The people of the town regret not having learned enough of their language. The elder people, who did not have the opportunity to learn French in school, feel guilty for not making an effort to learn it. The students, who did not take their language lessons seriously, regret not paying attention to their language classes.

The reason for this sudden realisation of the importance of their language is the fear of losing it. The announcement of the Prussian rule makes them understand that they may lose their language forever. Also, the emotional attachment to their language and the regret for not learning enough have made them realise the value of their language.

In conclusion, the story "The Last Lesson" portrays the sudden realisation of the people in a small French town about the importance of their language. The fear of losing it, emotional attachment to their language, and regret for not learning enough have made them understand the value of their language.