Law cannot be abandoned by the president in Ukraine. The president has limited power. As a commander the president has power over army, state security (СБУ), general persecutor. The president also signs enacted laws passed by the parliament and senate . For this reason Zelensky's inauguration is delayed till late May , so both houses pass the law on language and Poroshenko signs while he's still acting president.In my opinion it's a deliberate step of Poroshenko's team - he wants to make the term of Zelensky as hard as possible. Will that law help to solve the problem of the eastern regions where they mostly speak Russian? Of course not. So Zelensky would have to abandon that law otherwise follow Poroshenko in popularity.