Thank you for sharing your idea! The only thing that the global language setting does is to set the default value of the lang attribute that’s added to pages. You can change the attribute on a specific page by selecting the HTML element and editing the lang attribute. I don’t think a setting in Page Properties would be easier than that, but we could add it if others think it could be useful.
Searched also for this option and expected this feature more in the properties of each page (right-click on the .html-file) rather than in the HTML code itself.
Would be nice to have an icon for the properties in the title bar btw…