Here in England our plumbing is now metric, but as I understand it, the French, who used to buy their plumbing from England are still using imperial and have the sense to keep it that way, rather than introduce a second system.
Sort-of
In the UK we use a fairly restricted range of sizes - I suspect most domestic stuff was covered by 1/2", 3/4" and 1". These were internal measurements so when we moved to metric sizes the closest were 15, 22 and 28mm - these being outside diameters. You can throw in 8mm and 10mm (outside diameter) "microbore" pipe sizes for modern installations.
For threaded joints we use "British Standard Pipe" threads, commonly found on compression joints etc.
The French use a wider range of pipe sizes - basically anything from 8mm OD up in 2mm increments to 22mm then 28, 32 and 40mm. Their pipe tends to be 1mm wall thickness as well compared to 0.7mm typical in the UK. They quite like to hard solder (braze) their joints although there is regional variation and soft solder (lead free) is acceptable - especially for DIY.
For threaded connections they do, indeed, use BSP threads but they seem to use a much wider range of fittings than is typical in domestic UK installations.
So the actual pipework is all metric but the threads on some connections are imperial.