My goal is to create 3D meshes without the use of a 3D editor. So I want to know what representation languages and formats there are that are designed for this or otherwise that would be good for this. I would prefer one that is open and widely supported. Something like a 3D version of SVG or Postscript would be ideal.
There are a number of formats that might fit the bill. Depends on what you want to achieve.
Most likely your looking for a scene description language for renderers. Many of them are for that one renderer but at least one is a standard. So you might be looking for something like:
RIB, Renderman Bytestream although it comes with as a programming api as well. This is a based on a standard set by Pixar and is used by a number of render engines some of which are open source.
Most other rendering solutions also have textual formats I'm somewhat familiar with the mi (mental ray text or binary stream) formats and the now archaic Povray formats. But many others exist.
It is a bit debatable whether these really are human readable but given that you specify SVG then I guess they are at least and readable as that. Just more complex. Note like SVG these describe scenes.
Then we have
dae also known as collada. Which is sort of human readable has a similar niche as
RIB but is aimed for game assets. A number of other somewhat similar formats exist like the mostly deprecated VRML format, and its successors.
Then we have a bunch of polygon formats, many of them are binary. But atleast
ply are human readable and widely accepted. Obj is a very simple format. Although polygon meshes are seldom really easy to understand or craft manually if they are more complex.
Then we have a bunch of application specific formats. For example the
ma, Maya ascii, format which is human readable if you understand how maya node hierarchy works. Cant say much of the others.
Then we have the CAD transport formats
STEP. I wouldn't go and claim either to be human readable. But I have hand crafted both on a occasion. Both of these are ISO standards, although STEP is insanely big and was in fact the biggest of ISO standards in page count last time I checked.
There are quite many of these formats because 3D is not as uniform as 2D is. But, yes, I have hand crafted all of these at one point or another. Does not mean i would consider doing so as a productivity tool. Unlike EPS or SVG, which I craft bi weekly by hand.
If I would have to guess your motives then i would say RIB is probably what you want. Or then your looking to use obj.