[Решено] Default English home page is overwriting all other languages

Background of the issue:
I am trying to translate the home page of my website. The issue can be seen at скрытая ссылка. I expected to see the Dutch, French, and German translations, but due to obvious reasons, we had to use a hack, so you cannot see the issue at the moment on our website.

Symptoms:
The default English home page is overwriting all other languages. Instead of seeing the translated pages, I am getting the English version again, as shown in this video: скрытая ссылка.

Questions:
Why is the default English home page overwriting all other languages?
How can I ensure the Dutch, French, and German translations are displayed correctly?