What is the theme of the lesson "the last lesson" ? What is the reason behind its universal appeal?

The theme of the "the last lesson" is to show that people realise the importance of their mother tongue/national language only when they are refrained from using it. We tend to take our language for granted.It is only once we are refrained from using it, we realise how important it is for our identity as  a citizen of a particular country. It also shows that the national language of a country is what binds its citizens together.