Better support for cross-language magic links

[gudzpoz]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

This PR implements the changes described #3811:

• If the user specifies lang via link://.../...?lang=some_lang, pass some_lang as the lang parameter to the corresponding path resolver.
• Fix link://filename/... to handle multilingual posts.

closes #3811