[Resolved] Secondary language seems to have crushed primary language in some places!

Background of the issue:
I was trying to maintain separate language content for my website. However, out of nowhere, my English pages now have Bulgarian words. The issue can be seen on this page: hidden link

Symptoms:
Bulgarian words are appearing on English pages, which were not there before.

Questions:
Why are Bulgarian words appearing on my English pages?
How can I fix the language mix-up on my website?