Interesting question. I don't know how to elegantly do that, but in theory, you can write some code to delete the en branch of all pages within that site. there is no built-in "disable this language for this site" feature, however
"In Page Edit mode, Tool dropdown -> Language Settings -> disable “en” , then it is not working as expected." What is your expected outcome? By disabling the language, editors are not able to translate content into this language. You can disable a language anywhere in the page tree, e.g. on the startpage of website 'C'. To me, this sounds exactly what you want to achieve?
We have a multi-site solution where we have multiple sites like A,B,C etc. Sites like Site A and Site B are using “en” locale and those are already live. And we are planning to go live third site C as well, but for site C we have multiple cultures enabled like “en-us”, “en-ca” etc. and we want to disable the “en” culture for this specific site C only.
We tried below things:
Please assist on this.