Preamble

Navigation Menu is a theme feature introduced with Version 3.0. WordPress includes an easy to use mechanism for introducing customized navigation menus into a theme.
The dashboard interface has made great progress for users (and newbies) from version to version.
Inside a theme, a navigation menu appears at a location, currently near the header. A menu has a structure made with menu items to redirect to webpage containing (as WordPress describes them) a static page, a list of posts in a category, of a target date, or resulting of a query…
In dashboard side, we find in “appearance/menu” page, the location settings and how to set the menu with his structure.
Note: A menu structure can be attached to a widget.

In multilingual context on a standalone WordPress install,
for developers, it is possible to create a virtual location for each language (as presented in child theme Twentyfourteen-xili or other previous embedded (bundled) themes of WP install).

A menu location contains
   one menu structure with
      various menus items.

But there are (huge) thousand of themes (free or commercial) following or not following the codex rules of development.

Since 5 years, on the long way to work in the thousands of lines of WP source, it was recently possible to use powerful pieces of code to create menu item like insertion points replaced by navigation list elements.

A multilingual navigation menu with xili-language with menus insertion point.

In the content strategy of the example, we decide that the contents of menu are not similar in each language.
First step: a menu (structure) per language
These structures should not be attached to a location to remain free to be used by menus insertion point.
Second step: a core menu attached to the menu location of the theme.
Third step: a menus insertion point and a language list insertion point inside the core menu.

Limitation: a menu structure associated to an insertion point must not contains another insertion points.

Some screenshots: