default language

Hello,

I already posted in the main support page the following question but then I thought it would be something dealing with our favorite Seo plugin!
The default language on my site is set up to English but some posts and pages are written in French.
Is it possible to turn the default language to French only for those posts and how to do it without going into a complicated multilingual plugin or multisite installation?
Here is one of those, as an example.

Thanks a lot!
Hazlo

https://wordpress.org/plugins/wordpress-seo/