Copy *.ja from the 'builtin' folder of TS2010 into the 'builtin' folder in TS12. There should be no need to overwrite anything - the files are named differently. Go into the TS12 launcher and add your TS2010 CD key. You can also do this with the Trainz addon packs too - add any keys you have for those too.
Note: There will be minor side-effects - one example is textures from TS2010 are likely to be rendered black in the new satellite view.