As previously mentioned - yes, for many, many years in most of the former Soviet Union, Mongolia and China. It is allowable under ICAO - and you can fit another altimeter if you want, or use an FMS that is switchable bewteen the different units. It's not that dramatic (you can cvarry a conversion table), though it has undoubtedly been a factor in more than one accident in the past..
Do remember as well, the good 'ol USA is the most ICAO non-compliant country in the world...., but I guess that's OK as you generally started most of it....
