Requested Feature

• Support links with a lang query param like: link://slug/my-post?lang=de.
• Support cross-language filename links like: link://filename/my-post.other_lang.md.

Related Area

(I don't know. I guess it's related to the core nikola.py.)

Do you want to contribute this yourself as a pull request?

• Yes, I have already written code for it: master...gudzpoz:nikola:magic-link-lang

Does this feature affect backwards compatibility? If yes, in what way?

No.

• With Nikola 8.3.1, a link like link://slug/my-post?lang=de causes Nikola to abort because lang (None or not) is already passed as a positional param:

  TypeError: Nikola.path() got multiple values for argument 'lang'
  

  So I guess it is backwards compatible.
• As for link://filename/my-post.other_lang.md, it produces invalid urls half of the time. And I think fixing it is not a breaking change.

Rationale and full description

Currently, many magic links in Nikola (slugs, filenames, etc.) cannot link across languages. (If I understand correctly, the only one that seems to support this is link:///absolute-path.)

• Since the _link function in templates has an optional lang param, it would be natural to allow using a lang query param in links, like: link://slug/my-post?lang=de.

  A typical use case for this may be How to use translated slugs/URLs on a multi-language site? #3801 when the user want to link to posts in another language without having to hardcode links.

• Even if the user tries to link with absolute filenames via link://filename/..., with SHOW_UNTRANSLATED_POSTS = False, Nikola will produce invalid links, because it uses the current language in filename_path when calling p.permalink:

  nikola/nikola/nikola.py

  Lines 1970 to 1976 in 1da7205

  	results = [p for p in self.timeline if p.source_path == name]
  	if not results:
  	utils.LOGGER.warning("Cannot resolve path request for filename: {0}".format(name))
  	else:
  	if len(results) > 1:
  	utils.LOGGER.error("Ambiguous path request for filename: {0}".format(name))
  	return [_f for _f in results[0].permalink(lang).split('/') if _f]