Thanks for your fast response.
Yes, the backup was for the complete database.
I looked at both the .htaccess and wp-config.php files, and they both appear to be unchanged.
The wp-blogs table has entries for both the regular site and the French site. The only odd thing there is that the regular site has a last_updated date of today and the French site, of 2015-03-19. Maybe the latest WP update somehow missed that.
In any case, I noticed that the home and siteurl entries in the wp_3_options table were both set to https://le-vieux-manoir.com instead of https://le-vieux-manoir.com/fr/, so I changed that, and everything seems to be OK now.
Would that make sense to you? Does it seem like a fix?
Thanks again for your very prompt help.