Translation:There are many people in this bar, where we would drink beer.
It is indeed mixing tenses, but it doesn't sound weird in Dutch. The suggestion is that we planned to drink beer in this bar on beforehand, but now we see that it's busy, so we change our mind.
I think to convey the idea you describe in English, one would more likely say, "There are many people in this bar where we would have drunk beer." Using "would drink" sounds a little old-fashioned to me.
I also find it a rather strange sentence. It could be one that is thought by a group member entering the cafe. But it is definitely not a sentence that is spoken aloud. That would sound odd.
I agree with you it's an uneasy translation. It only really works if you translate as: "...would have drunk beers". "Would" in the sense of wanting or intending sounds quite archaic. I translated as "...were going to drink beers". "Surprisingly enough, the: "were going to" part was accepted without comment, but it was marked wrong, as the accepted version was: "...were going to be drinking beers". Personally, I can't see any sound argument for why: "...were going to be drinking" should be acceptable, but: "...were going to drink" not, so I have reported this.
"There are a lot of people here in this pub, where we wanted to have some beers." -- is probably the closest translation to this sentence using the order of its existing clauses. In English one would probably say, instead, "We wanted to (OR were going to) have some beers in this pub, but there are a lot of (OR too many) people here [so we'll be going somewhere else]."
Yes - the only other way "would" would make sense is as a way of saying "used to". So maybe they haven't visited the pub (where they used to drink beers) in a long time, and are disappointed to find it now so crowded. However, I'm quite sure the Dutch sentence is not reminiscing about the past, but expressing what they wanted or intended to do here and now (but presumably now won't).
Good point! Like saying, "There are a lot of people in this bar, where we used to drink beers." Implying that the bar is trendy now, but wasn't so in the past....
Why is it plural beers in Dutch, but when I use the plural in English it's wrong?
The plural should be OK in English too. We talk about beer as an uncountable OR a countable noun. To stay as faithful as possible to the original, I think it should be plural here. I only just noticed that the given translation isn't. If you're sure you haven't made a typo anywhere else in the sentence, report it.