i just thought that, since all the inter-lang links
are stored in the
database, so either the links exist or not, I don't know why there
will be one-way link.
They're not stored directly in the database, they're stored in the
source code for the articles just like links within one wiki. The only
way one wiki code detect the links to their article on other languages
articles would be to read every single article on that wiki. Not
practical.
The bots do quite a good job on the large wikis, I think.