My Irish father served in the French Foreign Legion 1950-1955. He fought in Dien Bien Phu and the Battle of Algiers. He never claimed his French nationality and died in 2000. Do I have any claim to French nationality?
This French Foreign Legion webpage says that a Legionnaire may apply for French citizenship after three years' service, or at any time after being "wounded in operation."
Assuming the same rules were in place in your father's time, he could have applied for French citizenship after three years of service. Because he did not, it seems unlikely that his offspring would be able to do so now.