The plugin that you have indicated seems to be made for switching more specifically between multiple languages, but I aim to set an unique language (which is not the default en codet).
I have already changed the configuration in the user/config/site.yaml file as you wrote, but this parameter seems to not been used as expected: the main language continues to be displayed as <html lang="en"> in the code source.
As the theme (Mediator) that I am currently trying to set up is using the same the same variable as the Antimatter default theme (i.e. <html lang="{{ grav.language.getLanguage ?: 'en' }}"> in the file named base.html.twig), I have some difficulties to see if this problem is originated from the theme, from my settings or from the main installation of Grav.
Yes, I did. But this option adds the supported language codet in the URL (i.e.: http://yoursite.com/fr) as described in Active Language via URL in the documentation.
It is not the behaviour that I expect: I simply want all the content of my site uses only another default language than en.
As far as i understand the documention that you mentioned above it is possible to choose a language manually by adding the language code to the URL if you have more than one language.