Villers Cotterêts Edict - French Language
Between 10 and 25 August 1539, King François I dictated a legal text known as the "Villers Cotterêts edict" comprising two hundred articles. To this day, two of of these articles have never been amended, making them the oldest laws still in effect in France nearly five centuries later.
The two articles set in stone the French language being used for all legal texts as opposed to Latin previously. The legal texts were Kings' edicts as well as church registries having to do with birth, death, marriage records, etc.
The language is one of the essential components of a culture and therefore a civilisation, so this is a milestone in affirming a French identity, separate from the Latin culture of south west Europe.