But leave them to indulge in idle talk and play [with words] until they face that [Judgment] Day of theirs which they have been promised:

So leave them to babble and play (with vanities) until they meet that Day of theirs, which they have been promised.

So let them flounder (in their talk) and play until they meet the Day which they are promised.

So let them talk and sport until they meet their day which they are promised.

So leave them to converse vainly and amuse themselves until they meet their Day which they are promised.