How do I know which language a Web site / Web page is written in? Are there special software or browser extensions to understand what language the website is written in?

Answer 1, authority 100%

Here is the extension:

https: / /chrome.google.com/webstore/detail/wappalyzer/gppongmhjkpfnbhagpmjfkannfbllamg?hl=en

With it you can find out what the site is written on. Good development 🙂
UPD: here’s another one:

https: // chrome ” .google.com / webstore / detail / similartech-prospecting / jiabgmelnfhgjkfdaoiccfcbaedjfcnm? hl = en

I advise you to use these two extensions, as they complement each other.
If interested: https://lifehacker.ru/wappalyzer/