For white collar workers (including software/it people), the law was implemented with a certain number of extra days/weeks of vacation per year, above the standard 5. And each company had it different.
So if one talks about the 35 hours law in France about software/it workers, please do not start counting weekly hours. Ask about how many weeks of vacation instead.
Accordingly, when I moved from France to the US, for the same company, i lost a few weeks of vacation, but boy, the days were WAY shorter.