Site Language does not appear in Geneal settings of each site

What you need is a language folder for your WordPress installation. Please have a look here: WordPress in Your Language

There is a language folder with all the languages in it in the multisite language switcher folder in the plugins folder on my FTP. I have no idea why I still do not have any options for language…

Yes, I know but there is no language folder in /wp-content. Create a folder languages in /wp-content and copy the language files I indicated above into that folder. After that you can choose the language in Settings > General.

Read on here please: https://github.com/lloc/Multisite-Language-Switcher/wiki