This can probably be considered fully translated now, if it wasn't before, so I'm retagging accordingly.

If anyone can explain the cultural significance (if any) of the yoghurt then please do. I admit I don't quite "get" it all. I'm guessing the yoghurt comes with a sugar sachet that you're meant to add to suit your own taste, and 'Lia has used it all and she's going to finish it herself.

One possible fact I can suggest is that Beatrice is sweet tooth. Bulgaria-brand yogurt/yoghurt in Japan is not sweetened, if I remember correctly, and she used all sugar to sweeten the yogurt, possibly one of few reason why she's angry.

It's also quite huge amount, as you can see. It's about good 500ml worth.