It's thought to be part of a larger series of sound shifts that happened as Proto-Germanic branched off from Proto-Indo-European, called Grimm's Law[1]. Romance languages like French (along with other Indo-European languages, like Greek and Sanskrit), as far as we can tell, preserve more closely the original PIE sounds, with Germanic languages being the 'deviants'. It's also the reason why English has 'feather' and Romance languages have 'penna' or some derivative, and there are a bunch of other fun examples of cognates to be had.

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[1] https://en.wikipedia.org/wiki/Grimm's_law

Of the Grimm Brothers, the same responsible for Grimm's Fairy Tales. Jacob was a philologist in addition to a mythologist.

«Of the Grimm Brothers, the same responsible for Grimm's Fairy Tales. Jacob was a philologist in addition to a mythologist.»

It's fun because the myth collection was almost a side effect of the linguistics work: it shouldn't be a surprise that when the Brothers Grimm walked into a small village and asked for the oldest documents the village could find they got a lot of interesting myths and fairy tales in return. There's a bit of dramatic irony that Grimm's Fairy Tales has had such a homogenizing force on the myths since its publication when a lot of the reason for collecting them in the first place for the Grimms was seeking all the little nuances and differences and distinctions between them (including and especially linguistically).

Many thanks :-)