DSMJ system will only work DSMJ receiver and TX.
DSM/ DSM2  system will only work DSM/DSM2 receiver and TX.
So, This mean , DSMJ and DSM are different and are not compatible.

The reason why the regulation for 2.4Ghz different is each country has different law for 2.4Ghz.
USA version is too strong in Japan or Europe.

You can think of DSM2 USA and DSM2 Europe as brothers.  Virtually the same, just the frequency is a little stronger in the USA.