How can I make the url always have the language parameter?

Question

PHP

Hello!
I have organized multilingualism, and the language is written in cookies lang = en
when I write site.ru/lang=ru in the URL, then it is written in cookies lang = ru and the site works in Russian!
but then the parameter in the url disappears!
How can I make the url always contain the language parameter?, for example site.ru/en/catalog.php or site.ru/ru/catalog.php etc.

Answer 1

what engine is used on the site?

Answer 2

Once self-written, then simply improve the pages.
Something like this: in the ru folder - Russian pages, in the en folder - English

Answer 3

I would do this:
Open the page in English - all links lead to English pages