Why go with modern France? Let's think along the lines of Ancient France/Gaul/Frankish Kingdoms!
Name:Fanaticaul
Noun: Fanaticauls
Adjective: Fanaticauloise
Name: Fanaticallia
Noun: Fanaticelts (last syll. pronounced 'kelts' not 'selts')
Adj: Fanaticallic
Name: Fanatickish Kingdom/Empire
Noun: Fanaticks (add Merolvingian or Carolingian before noun if desried)
Adj: Fanatickish
Name: Holy Fanatican Empire
Noun: Holy Fanaticans (throwback

)
Adj: Fanatickish OR Holy Fanatican
Name: Fanatis (pronounced FanatEE)
Noun: Fanatisians
Adj: Fanatisian
And before anyone decides to remind me that the Franks (a la the Frankish Kingdom and the Holy Roman Empire) were Germans, let me say that it was France, not Germany, that ultimately kept the Frankish name.
