How can i display a full language description ( i.e. English instead of en)

First of All, Thank you for the great plugin. I need to implement the language in the header … and it’s work very well, but the languages display in two characters for example en, ar … etc … what i need is to present the language in full description (i.e. English, Arabic, Spanish…), can you please describe me what is the way to do this !!

https://www.ads-software.com/plugins/multisite-language-switcher/