I think this plugin is not necessary anymore:
http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/
(If you want to still link it, it can be shortened to https://wordpress.org/plugins/wordpress-mu-domain-mapping/)

Codex articles have moved:
https://codex.wordpress.org/Create_A_Network to
https://wordpress.org/documentation/article/create-a-network/

http://codex.wordpress.org/WordPress_in_Your_Language redirects to https://make.wordpress.org/polyglots/teams/
Not sure if this is the article you want ...

https://github.com/danieledesantis/jquery-language-detection is 9 years without updates
https://github.com/oncleben31/Multisite-Language-Switcher-Theme is archived/read-only since 2020.

Redirecting is a bad idea, as Google is following these redirects which could impact your indexing very badly:
https://wpml.org/documentation/getting-started-guide/language-setup/automatic-redirect-based-on-browser-language/how-browser-language-redirect-affects-google-indexing/

Unfortunately Network Shared Media is closed.

Maybe these two could be helpful:
https://github.com/bueltge/multisite-global-media/
https://github.com/humanmade/network-media-library