Normally, 666 is seen as the number of the beast (well it's not, it's actually the mirror image to not invoke any evil influences by refering to 666 just in case) so I've searched for battles in 1666; I was only shown battles from the second english-dutch war.
I've found for (probably) french (or romanic language) speakers:
-) Job Forant (1606/1612/1630 - 1692) (and yes, this are three birth dates for him, seemingly there are three different sources for his birth from the french wikipedia side; I didn't found information about him on the french or english one)
-) Michiel de Ruyter (1607 - 1676) (though I'm not sure he was a native french speaker)
-) Cornelis Tromp (1629 - 1691) (though he learned french later in his life)
-) Abraham van der Hulst (1619 - 1666) (pretty sure his native language was dutch)
-) Jacob van Wassenaer Obdam (1610 - 1665) (see above for language)
-) Cornelis Evertson (1610 - 1666) (see above)
-) Pieter de Bitter (1620 - 1666) (see above)
-) Cornelis de Witt (1623 - 1672) (see above)
-) Johan Evertsen (1600 - 1666) (see above)
-) Willem Joseph van Ghent (1626 - 1672) (might've been native french speaker)
-) François de Vendôme, duc de Beaufort (1616 - 1669)
-) Gilles de La Roche-Saint-André (1621 - 1668) (who surrendered against the british due to a mistake but was honourably returned immediately by King Charles II, the Duke of York offered him a sword before his repartiation)
All of these don't fit the criteria with their time of life.
I tried to change my search and got through the whole year of 1666 ( https://en.wikipedia.org/wiki/1666
), there was some thing about the portuguese restoration war, which included:
-) Diego Ibarra (??? - ???) (a basque officer fighting for Spain in the battle of the berlengas)
-) António Pessoa (??? - ???) (a portuguese soldier in charge of the fort São João Baptista, fighting against Diego Ibarra)
-) Frederick Schomberg, 1st Duke of Schomberg (1615 - 1690) (marshal of France, fighting on the side of Portugal) (born in the Palatinate/Heidelberg so most likely not a roman language native speaker)
-) Peter II of Portugal (1648 - 1706) (though he was kinda part of royality so probably not)
Other than that, I really don't know what the hint would point at; especially, since under the rubric 'Date unknown', the entry of Isaac Newton splitting sunlight with a prism has more accurate and supplemental information than the portuguese restoration war entry.