Just an update on the preview: yesterday I figured out that the translated pages are generated only for those languages that are listed in the website’s config file. I set this up on purpose for our own convenience. But this means that for pull requests those translated files are not available, and thus any preview would have the pages for the new language (here: French) in English.
Long story short: I will have to think a bit further how to get translations in to the preview, so it’ll be somewhere next week/weekend. Sorry for that.