https://www.quora.com/What-is-the-plural-form-of-the-word-beer It would appear that it depends on whether we're talking beer as a liquid in general, or if it's in reference to bottles of beer. Although in this particular sentence I would agree in thinking that 'the beer' sounds better vs. 'the beers' (I suppose it's like 'fish' vs. 'fishes'. You would use 'fish' for multiple individual fish, whereas 'fishes' is more used for multiple species/varieties of fish. http://grammarist.com/usage/fish-fishes/)
Should it not be the mouse is not drinking the beer, Beer can be either countable or non countable, is it the same in Danish?