"We do not know if this is true."
Translation:Ми не знаємо, чи це правда.
No, that's direct translation from English... We use якщо only for saying conditions like "if, in case of" and чи is for "if, whether, is it so or not"
i.e. ми не знаємо якщо це правда sounds like "If it's the truth, then we don't know"
I see you are familiar with German, чи = ob