What’s an easy way to generate a font asset with all the characters required for the EFGIS language pool? How do you guys get the ranges? Does anyone have a convenient list somewhere living on the side? 
By EFGIS, you mean English, French, German, Italian and Spanish? If so, Extended ASCII should pretty much cover you. You can always check with Unicode chart.
Having said that, the easiest way to handle localization now (in version 1.4.0 for Unity 2018.3 or 2.0.0 for 2019.1) is to generate a primary font asset that contains Extended ASCII and then assign a fallback font asset of the same font set to dynamic mode.
Oh, rejoice! The dynamic mode for a fallback font is exactly what I was looking for. Thanks!