Not Japan, but I read somewhere that the vast majority of bicycles in Vietnam are mixtes, Of course, from the mid-19th century until just after World War II, Vietnam was French Indochina so that might be somehow related.

bastiaanson has done an excellent job explaining the pronunciation. 'Meext' is the easy bit; it's the 'e' that's hard to describe - like the 'e' in Porsche except not the way Janis Joplin (God rest her soul) pronounced it in 'Mercedes Benz' - "Ma friends all drive Porscheys, ah must make amends"! It grinds my gears, though, when hipsters pronounce it 'mixty' - it's only got two syllables and they manage to get them both wrong.

Incidentally, way off topic but re. Japan and pronunciation, I remember (I remember odd stuff) that somewhere on the forum noglider explained, even posted audio, that 'Tange', as in Japanese Tange tubing, is pronounced "Tan'geh" with a hard 'g', not "Tanj", which is the way I always thought it was pronounced.