During any wajib salaat, in Qunoot are we allowed to also recite the supplications in our own language or we are only supposed to recite in Arabic only? Same question applying to Salatul layl. Can one also recite a short supplication in their own language during the qunoot in namaaz witr or is that Qunoot only for Dhikr and forgiveness and should only be recited in Arabic?

Only qunoot is the place where you can recite in your language or in any language you want to pray , there is no difference in wajib or sunnat prayer.
Yes you can recite dua and seek your hajaat in your own language in wajib prayer as well in salatul Witr.