How to fix language switching on the site?

Question

Internationalization and localization

Hello.
Made the site multilingual (site.com/ru/, site.com/en/).
I use links as a language switcher, but the bottom line is that when you go to them, the site changes localization only after the second click on the link. Because the first time you click, the site is updated, but the language does not change. And after the second click, the site is updated (as far as I understand) completely and changes the localization.
How to make the language change the first time?
PS The site is self-written.

Answer 1

And what is sent in the header of the pages and what are the server settings for these directories?