That is...very debatable. Even before the Protestant Reformation, France, Germany, and Spain had radically different cultures. Americans and Englanders are often surprised at how little we share in common in terms of culture and values, to say nothing of our more distant cousins on the Continent.
Then there's the fact that pre-Romans (even if we call them "Celtic", which not all of them were) and post-Roman (even if we call them Germanic) were really a collection of interrelated culture, not a cultural monolith, that were each quite distinct - a Frank was not an Angle nor either of them a Goth (and even Visigoths and Ostrogoths each went their own ways), and Burgundians and Lombards and so forth all come into it too...), so reducing it to "celtic" and "germanic" influence erases a lot of distinction.
They might as well be a cultural monolith compared to the differences between France and India, or France and China. The point of Culture Groups is that these cultures share common traits. I guess you could have many different groups instead of a 'Western' group (West Germanic, East Germanic, South Germanic, etc), but then that would limit the immortality of the player...actually this sounds like a good idea. Imagine if you were playing France in WWII and Germany kills you, you could then swap to America, gear up and K.O Germany.
And finally of course there's the fact that over the course of centuries cultures evolve as much in response to their local environment and their own history than in response to their roots, taking their own widely different directions along the way.
this is what i want,
I actually detailed cultural changes in a thread. The easiest way to make Cultural Groups is determine that to be in a certain group, you have to 1. Come from the same
recent origin (Proto-Indo-European splitting into Classical European cultures would not count for a Indo-European culture group, but British Isles Celtic and Gaul Celtic would qualify for the same Celtic cultural group) and 2. Share 1 or 2 or 3 values with a certain value (i.e, you must have 4 militaristic score, -2 libertarian score, and 4 expansionist score to be part of this Culture Group)
Ah, yes, because clearly France should be founding a civilization that was created by English colonies, instead of, you know, founding its own colonies *cough*.
honestly I don't know how to treat colonial nations