Add the BinaryUnitDialect=0 option to the [Locale] group of your $KDEHOME/share/config/kdeglobals file.
setting to 0 (or leaving unset, of course) gives you the current default of IEC units since they are unambiguous and in accordance with the current revision of the relevant standards documents. Setting to 1 gives you JEDEC units. Setting to 2 gives you metric (yes, real powers-of-10, lowercase k instead of K, metric units).
В GUI, увы похоже не прокинуто. Странность для KDE с его миллионом ручек и крутилок.