Does CHT=CHT have a geometric isomerism because of tritium atoms?
Yes isotopes can induce geometric isomerism.  You can induce chirality by using different isotopes of atoms; the molecule $\ce{CHDClF}$ is chiral with the deuterium atom (you do not need radioactivity) while ordinarily $\ce{CH_2ClF}$ is not.  Chirality won't work for ethylene (that molecule is still planar and thus mirror-symmetric about its own plane no matter what iostopes are used for any atoms), but cis-trans isomerism with two deuterium atoms still can work.