No, there are no French subs in either edition, French (Studiocanal) or British (BBC). It's a general problem with French films and DVDs, and our HoH subs wouldn't help foreign students anyway: they're not closed captions of the dialogues, but multicoloured texts that essentially summarize the dialogues. They're meant for deaf people who can at least partly read on lips, and French spelling is generally too long while pronounced very fast, so the text dialogues would be displayed too fast for reading. Then again, faithful subtitles in a show like Engrenages wouldn't really help a non-native speaker since it requires familiarity with the police lingo and various forms of slang. This is a tradition in the French cop film genre, something HBO adopted when they produced The Sopranos and The Wire. Most of the French cop films and films noirs (like Melville's) are subtitled in academic French on the French channels of Radio Canada, because French Canadians don't understand that slang either (as I doubt they can understand most of Spiral).
Thank you for your very swift reply. It is useful to know the reason why French subtitles never seem to be available. I have ordered the Spiral Box Set and hope I don't pick up too much slang! I shall no doubt need the English subtitles and am looking forward to watching the first one.