It might have changed since I last had to deal with this (I hope!) but excel, at least ~5 years ago, was storing cell contents as they appear in the function box in the UI. As in, `1,23` is 1 and 23/100 when read in fr-CA, and this would apply to the function names as well. So `=SOMME(...)`. Excel is smart enough to pick up on a locale flag in the file format, but obviously it then shows you a dialog asking you to convert it, needing the language pack.

I also won't assume that we were making "good" excel documents. It's possible we were shipping badly made exports haha