What would be the biggest differences today and in history if France remained Germanic culture/language rather than Romance?
Frankish Gaul wasn't really germanic in first place.
Ethnical Franks only represented a minority of the overall population (5 to 10 % in Northern Gaul, far less in southern parts), and as Roman elite and population (North of Loire, as Aquitains and Provencals kept calling themselves "Romans" up to the IXth century, and their language "Roman" during all the Middle-Ages).
Granted, German element still represented a good part of the population in Austrasia and (surprisingly) Merovingian Germania, but it was only a part of Francia (even by the more restricted meaning of the name, aka the region between Loire, Atlantic Ocean and Elbe/Danube border with Saxons) and critically not its demographic center.
Werent Franks a Germanic tribe?
Originally yes, but the name took a different meaning since the VIth century, basically designating all nobles north of Loire whatever their origin.
Bruno Dumézil quite recently summarized the process that lead to this mix, as "Gallo-Romans played Barbarians", naming their children with Germanic names, and with Frankish customs but actually romanizing Franks including in linguistical matters where Roman element was tronger (It can be argued that Franks established in Gaul since the IIIrd century were quite romanized already).
It can be safely assumed that the Frankish nobility of northern Gaul was at least bilingual before the IXth century, and by then it's certainly the case (troops of Charles the Bald using a romance speech, as highlighted in the Oath of Strasbourg).
But it wasnt only the aristocracy that was Germanic. They came there as a tribe so they had a people to call upon. I think it had more to do with their desire to rebuild Roman empire, than with demographics.
Actually, Franks weren't a people but a confederacy of peoples. It's usually divided into Ampsivarians, Bructari, Hatuari, Hamavi, Salians, Sicambri, Tincteri, Tubantès and Usipians.
And by the time they entered in Gaul, depsite being still divided, they were usually grouped among two groups : Salians and Ripuari (Rhenan Franks).
We're talking of a proteiform identity (as it was much the case of all western germanic confederacies and peoples), and of a really limited demographical group (estimated around 5% of the total Gallo-Roman population).
Conversion to catholicism was pretty much unavoidable at this point, and even if it was delayed, the fusion between GR and Frankish population would have happened eventually (as "Goths" designated since the VIIth century the overall Hispanic nobility).
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For France to be germanic, you'd need a completly changed definition of France (as in, Franks never settle and dominate Gaul, but instead remain in Germany). At this point, it's possible that they would either disappear from History , remain a marginal group (Frisians) or only last by giving their name to a german region (Saxony).