(Referral from a Facebook group where a series of judges disagree.)

If Teferi, Hero of Dominaria’s controller controls only untapped lands when the delayed triggered ability from its +1 ability resolves, must its controller choose and untap tapped lands that another player controls, if available?

(I realize that Teferi’s controller can just tap two lands whilst the ability is on the stack, but it is plausible that this may not happen.)

Yes. When the delayed trigger resolves, its controller must untap two lands if it's possible to do so. If the only tapped lands on the battlefield are controlled by the opponent, the player will be forced to untap some of their opponent's lands.