As a logged-out user:
- access https://fr.wikipedia.org/wiki/Aide:Flow. The context page is in French.
- click on the link: you access a page in French on Mediawiki.org (because you have forced the /fr page to be displayed)
- click on a link in the page content
The new page you access is in French.
The new page you access is in English because Mediawiki.org has English as the default language.
That problem breaks the workflow for users as this feedback reports (in French): people don't understand why pages are in English (and apparently don't see that they can switch the page language).