I would settle for
https://www.sample.com/en/
And not the current
https://www.sample.com/?language=english

Prior to whmcs 8.0, we were able to modify the htaccess file to make this work across all urls. However, after v8.0, this modification works for all urls, except any page that uses index.php. As of v.8, the homepage breaks and so do any pages using index.php

If you will not add this simple feature for multilingual sites (really needed for seo), then at least allow us to modify the htaccess like before to make this work on our own. This is where you can see the previous htaccess code which no longer works for index.php files. At least allow us to make this work: https://whmcs.community/topic/216997-change-language-without-languageenglish-in-url/