May or may not have some relation, but next to France/part of, lies the Basque country, where all numbers under 100 are base 20+10, except 11 and 19...

57: 2×20+10+7 (berr-ogei-ta-hama-zazpi)

79: 3×20+19 (hiru-r-ogei-ta-hemeretzi)

French (in Belgium, Switzerland, and former colonies) also allows simple base 10:

70: 70 (septante)

91: 90+1 (nonante-et-un)

...so the geographic location seems to have an impact.

And just next to it, in Spain, everything is base 10... except 11 to 15 change the order from n×10+m, into 1+10 to 5+10.

Italian does the same, except it's 11 to 16... just like in French.

English has a hiccup with eleven and twelve, then goes to n-teen, before going base 10 with n×10+m above 20.

German does the same, except it goes to m+n×10 above 20.

Overall, 20 seems to be a magic number, France just seems to have mixed in different ways of using it.

https://en.m.wikipedia.org/wiki/Vigesimal