On the mask task -- when the mask roadblock came up in Venice in season 4, it was regarded as the most difficult roadblock ever to that point. (And all the teams struggled with it). This version as a detour task seemed to combine to waltz dancing fast forward in the sense that the Racers had to work around the dancers to find the right masks, and the masks themselves were far more difficult and thus harder to match up. I don't think it was an easy task at all. The chandelier one probably wasn't chosen because the teams probably reasoned that it would be difficult to complete because of the requirement that teams would have to start over if any chandelier wasn't assembled correctly or fell to the ground before completing the attempt.
For the same reason I felt the singing task wasn't any less difficult than the piano tasks that had been done with a couple of variations in past seasons, or the other singing tasks that have shown up in different seasons as well.
I'm not sure that knowing a romance language (Italian, French, Spanish, Portuguese) would have been as helpful because German is used in opera and there are no soft sounds in German as they are in the Romance languages whether spoken or sung. When I was in a chorus in junior and senior high school years ago we did have to learn and perform a few songs that were in German, and for most people it was harder. But it helps to have studied German at the time, and so I could evaluate that task with experience. I'm not surprised some teams had more trouble with that task than others, and people who couldn't sing, and who weren't familiar with languages other than English could expect problems, especially with certain US dialects.