Okay, for what it's worth... I created a stub of "Central Nuit" and no, it's not found.
Querying thetvdb.com API directly, it returns nothing:
http://thetvdb.com/api/GetSeries.php?ser...ral%20Nuit
However, specifying the language, and it shows up:
http://thetvdb.com/api/GetSeries.php?ser...anguage=fr
QED Kodi needs to pass the language over when it scans.
When scanning it in, I couldn't find a way to force it to FR (e.g. you can't change the name to "Central Nuit (fr)"). However, going into the addon settings for that source, you can select a new language there. Change this to FR and re-scan, and Central Nuit immediately appeared in my library.
I then changed the scraper back to EN (my default), and added a fake episode (Central Nuit - 1x01.mkv). I then scanned for new content, and it was found and inserted into the library as "Nuit de chien".
So there's your answer... the addon needs the language settings to know where to look for the series initially; however, once it's found it, it doesn't need that language again, so you can change it and add episodes as normal.