Russia and China use metres.
Feet for altimetry is probably a historical thing. Yanks kicked off aviation and they used it..
What's stopping you? Well, no one else in European airspace is using it and unless your avionics automatically convert it, you'll be forever looking up conversion charts when you should be concentrating on other stuff.
Actually not true except for pure IFR traffic
Gliders and ultralights at least in germany but probably in france and other countries in europe as well use metric units. I learned to fly that way (of course started flying at 14 in glider planes) and had to relearn quite a bit when i started my ATPL studies. Modern glass-cockpit style instruments offer both unit systems or can be easily switched between both.