People might be surprised by this, but as far as I can tell such Nutritional Facts labelling didn't start being required on packaging in Europe until the late 90's, several years after the full mandate in the US and nearly a decade after the original proposal in the US.
Nutrition facts per 100g is conceptually nice, but if the amount people typically eat at once is not 100g, then that means more math. Furthermore, in the US most recipes are not done by weight, so to be useful for recpie calulation, we would need to use say per 8 fl oz for liquids, a per
The problem with the US system is more that the FDA implements it based on survey results for categories of food, and only occasionally update the value. This means that food that falls in one category, but that consumers treat like it falls in a different one can get laughable portion sizes, and the same thing happens when the values become out of date.
Manufacturers don't really get a lot of discretion on choosing the listed serving size for most foods, with only a few places where the rules let a manufacturer choose (like for discrete units that take more than half and less than two thirds of the RACC, where the manufacturer can choose between a service size of one or two units). There are cases where manufacturers would prefer to list a different size (usually larger) because they know it is more accurate representation of what people consume, but they can't.
This is hardly the only issue. Some of the rounding rules can be rather absurd, meaning that is is not always possible to get accurate information for larger sizes by simply multiplying the values on the nutrition facts label.
Do you or someone else happen to know which directive made it mandatory? I found Council Directive 90/496/EEC of 24 September 1990[1] which harmonised labeling, but only made it mandatory when there's a nutrition claim on the label/advertised. Otherwise it was still optional.
[1]: https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:31...
but as far as I can tell there was no obligation in Europe to have the level of detail about fats, sodium, etc. like the US's labels had until even later- maybe 2006? [2] and there was the last major change it seems in 2011 which is what is still essentially in place today. But yeah, it seems like the level of information made mandatory and not optional wasn't like it is today until around or maybe slightly after the Millenium.
[1] [PDF] https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:...
[2] https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:02...