To delete a language, go in Languages > Languages. Click on the red delete link for the language that you want to delete. You will get a confirmation message, however your choice is definitive.
How your content is impacted?
❶ Before deleting the French language, both posts have a language and are translation of each other.
❷ After having deleted the French language, the French post (Post FR) is not removed but no more language is assigned to it. You also loose the link with the translations in other languages. The posts without language are still available and visible on admin side, but they are not shown anymore on frontend side.
Pages, categories, tags, custom post types and custom taxonomies are impacted in the same way. The navigation menus are not deleted either but no more language is assigned. Strings translations are definitively removed. The same for all users biographical information in that language.
Once a language is deleted, you will get a message above the languages list table to warn you that not all content has a language assigned (and thus that not all the content is visible on frontend). The link offers you the possibility to assign the default language to all the content without language.