Do years before Christ (or zero) follow the same reading pattern as here? How do we say and write B. C. in German? Do they have other forms of expressing that (e. g. non-religious ones)? What about other calendars (Jewish, Eastern Orthodox)?
There is no year zero. (This is independent from the language)
Das Jahr vor dem Jahr 2 nach Christus ist das Jahr 1 nach Christus.
Das Jahr vor dem Jahr 1 nach Christus ist das Jahr 1 vor Christus.
With the usual abbreviations:
Das Jahr vor dem Jahr 2 n. Chr. ist das Jahr 1 n. Chr.
Das Jahr vor dem Jahr 1 n. Chr. ist das Jahr 1 v. Chr.
The year before the year 2 AD is the year 1 AD.
The year before the year 1 AD is the year 1 BC.
But note that in astronomical calendars there is a year zero, but there are also negative years:
The year 2 AD is the astronomical year 2.
The year 1 AD is the astronomical year 1.
The year 1 BC is the astronomical year 0.
The year 2 BC is the astronomical year -1.
The year 3 BC is the astronomical year -2.
The German terms vor Christus and nach Christus and their abbreviations have of course a religious connotation, but since it is the most usual form, this connotation is weaker than it might sound to learners of German. I am an atheist but I still use these expressions because they are so common and I don't have a problem with it, like almost every second German native speaker who is non-religious or member of a non-christian religion.1
There are also other terms and abbreviations, but they are so rare, that most people don't know what the abbreviations stand for:
- vuZ = v.u.Z. = vor unserer Zeit = vor unserer Zeitrechnung
- nuZ = n.u.Z. = nach unserer Zeit = nach unserer Zeitrechnung
For the numbers themselves:
Everything that was explained in the answer you linked to in your question, is true also for dates before Christ. You just have to add »v.Chr.« The rest is the same.
For dates in other calendars:
I think you do it in German the same way as in English (how ever this might be - I don't know).
Generally, the years B. C. are read in the same way as are the years A. D. For example
Romulus und Remus waren nach der römischen Mythologie die Gründer der Stadt Rom im Jahre 753 v. Chr.
what would be read as "(...) im Jahre Siebenhundertdreiundfünfzig vor Christus".
The equivalent for "A. D." would unsurprisingly be "nach Christus" or "n. Chr.", though this is very often ommitted. We rarely speak of "das Jahr 2023 n. Chr.", unless there's a specific reason for that ;)
Additionally, there's a way that tries to avoid religious connotations, and that's referring to "our way of calculating times", "unsere Zeitrechnung". You can compare this to "before or after the Common Era". With this, the legendary foundation of Rome would have been at
753 vor unserer Zeitrechnung (v. u. Z.)
For "A. D." we might say
das Jahr 2023 unserer Zeitrechnung (u. Z.)
das Jahr 2023 nach unserer Zeitrechnung (n. u. Z.)
You may want to note that "nach" in this case doesn't refer to "after" congruent to "before" in "v. u. Z.", but more to "according to". "Nach unserer Zeitrechnung" would be something like "according to our way of calculating times".
As an aside, in our calendar there was no year 0. After the year -1, so to speak, the next year was the year 1. That's seen from today's perspective, of course, the people of the time used different calender(s). You occasionally find people using a year 0 in their calculations, which means they determine the date for the coming apocalypse one year wrong ;)