I could elaborate this Problem in more Detail. Here’s how it can be reproduced:
1. Create a new WP Installation, install and activate PolyLang, activate the Theme “Twenty-Sixteen” (Default)
2. Create two new Pages, for example titled “Intro” and “Blog”
3. Change Theme Settings: Static Start Page: “Intro”, Blogroll: “Blog”
— At this Moment, everything works fine. The static Pages are displayed, the Blog Page shows all Blog Posts —
4. Go to the Polylang Settings, choose “Settings” and select the 2nd Option for the URL Modification to show the Language in the URL, for example: https://www.example.com/en/
— The Blog Plage is now displayed as static Page, not as Blogroll. —
5. Undo the last Settings change in Polylang
— Everything works fine again —