Bonjour Marc,
I can add some information for her greek carrer, but haven't found anything on the use of DELFIN by the french Navy.
26.08.1912 she was commissioned in the Royal Hellenic Navy
30.09.1912 – 05.10.1912 she was brought from Toulon to Piräus
During the Balkan Wars:
-on 22.12.1912 at 10.45h she missed the ottoman cruiser MECIDIYE with one torpedo off Bozca Ada, Dardanells (as far as I know the first torpedo fired by an submarine against an real target).
- at the End of January 1913 she had to do patrols in the Gulf of Athens together with torpedoboat NIKOPOLIS against the ottoman cruiser HAMIDIYE
In May 1915 during exercises with the other greek submarine XIPHIAS and torpedoboat NIKOPOLIS she suffered slight damage due to diving deep after an handling error by her crew, she was under repair until July 1915.
You can find some additional information here:
http://www.mezeviris.gr/admiralnarrations.html
than click on "happy days 1913 - 1915"
It's an great site about the life of the greek Admiral Mezeviris, who at that time (May 1915) was CO of the DELFIN!
She was seized by the French Navy in December 1916, in 1917 she was under french flagg but made no patrols, in 1918 she was returned to Greece.
Cordialement
Oliver