Polish justice system distinguishes between więzienie, which is a long-term confinement facility for convicted criminals, and areszt, which is used to detain suspects.
From what I understand, in English prison is więzienie and jail is areszt.
Dlaczego twój szef jest w areszcie?
About the difference between "in jail" and "in a jail", or "in prison" and "in a prison", Polish doesn't distinguish those, although you can use verb siedzieć instead of być to refer to someone who is detained compared to someone who just happen to be in there:
Dlaczego twój szef siedzi w więzieniu? – Why is your boss in prison? (=What crime has he been convicted of?)
Dlaczego twój szef jest w więzieniu? – Ambiguous –the boss can either be sentenced or just visiting, or be there for any other reason.
Yes, I mean Polish movies of course.
Well, the thing about capitalization is as follows: yes, it is polite and recommended to capitalize all forms of "you" and "your" (not only Formal You, all of them) when you use them while addressing someone directly. So for example in your comment above there's no reason to capitalize them. But if you were writing to someone, let's say, "Co u Ciebie?", that asks for a capital letter.
Duolingo sentences don't use them, as they are just sentences taken out of context. Even though many of them address some imaginary interlocutor, this is not a real address to a real person. It's like a line from a dialogue in a book. And there is no capitalization in dialogues. But writing to someone (letter, comment, text message, Facebook message) - yeah, you should do it there :)
I have to ask, I am also studying French and German on Duolingo, and have noticed a difference in the tone of the phrases I am learning. Only in Polish am I learning phrases such as: "Twoi żołnierze są słabi" ( Your soldiers are weak), "Mój kraj nie ma dla mnie pracy" (My country has no work for me), or "Dlaczego twój szef jest w więzieniu?" (Why is your boss in jail?) Is it me, or does the Polish course have a somewhat darker edge to it's phrases? I asked a Polish friend and I think he's a bit offended now
I can't speak for your friend, but to me, there is nothing offensive in your observation. Different courses are created by different people and their contents aren't necessarily determined by the general mentality of a nation, but rather by the mindset of a few individuals who came up with those sentences.
However, it has indeed been observed that Polish people are more likely to skip small talk and engage in rather serious conversations about social and political grievances than their western neighbours. Being sceptical and addressing problems directly is definitely something I would call a common trait among Poles.
Having said that, only very few sentences in this course have this noiresque vibe and there are also quite a lot of humourous and light-hearted ones which act as a counterweight. All in all I believe this course to be a quite decent representation of the versatility of Polish culture.