Multilanguage plugin: support x-default

[oscarotero]

Enter your suggestions in details:

The language marked as default could be added as <link rel="alternate" hreflang="x-default" href="{{ url }}"> in the head.

https://developers.google.com/search/blog/2013/04/x-default-hreflang-for-international-pages

[ngdangtu-vn]

Reference:

• https://webmasters.stackexchange.com/questions/103610/how-to-use-hreflang-x-default
• https://www.hreflangbuilder.com/complete-guide-to-using-the-using-x-default-hreflang-element/

This open to 3 cases:

1. A external (lang route) link or a specific page for choosing language
2. A fallback link (could be internal or external)
3. Not use

I wonder if I miss anything :?