Wiki says they (pipe sizes) are "international". But surely not. How about China, (say) ?
Imperial used in China like everywhere else.
Imperial is used for thread sizes all over the world because Great Britain invented modern plumbing. Once upon a time pipe was either threaded or joined with lead solder wiped with moleskin. The advent of pre-soldered joints, a British invention by the Yorkshire Copper works in 1934 led to copper tubes in Imperial sizes replacing iron and lead pipes.
When the UK succumbed to metrification, our USA cousins didn't, the pipe sizes changed from 3/8 to 15mm. and 1/2 to 22mm. etc. However, the Europeans couldn't fiddle the thread sizes as it would cause chaos so they have been left alone.