I can see your POV, but I still think you're being unreasonable.
I mean, I get your desire for privacy, I barely like sharing with one other person let alone someone outside of an intimate relationship, but when you advertised for an au pair, you knew you were sacrificing some privacy in exchange for extra help. It's just as daft as someone taking in a lodger to earn some extra money and then complaining about someone else sharing their communal space.
And I get that lockdown means your expectations and reality are not matching up. Normally, no one would stay in the house all day every day if they avoid it!
From her POV though, what you are essentially tiptoeing around is that she's good enough to do the things you don't want to do, and she's good enough to look after your most precious possession in the world, but she's not good enough to share some time with. Can you imagine how upsetting that would feel to know that when you're all locked down in a strange country and can't escape?
I wouldn't want to spend all my time in the sitting room, but she apparently does, so she's probably lonelier than you think.
What arrangement do you have with her? It's not uncommon to pay an au pair a salary and also pay for language classes for them. If you were to set up a Zoom language class, she could have more human interaction, improve her English (I assume that's something she's looking to do, it's why most people au pair in a different country) and you'd know there was a fixed time each week when she would be doing something else.