Disabled language was seen from some PCs

Hello All !

There is something very strange with my OpenCart.
After reading and self estimation, I found that I cannot resolve the issue.
Please, advise if you have a little idea what magic was happening here...
Here are the facts:

OpenCart intallation 1.4.9.3 with default ENG language.
Added BG language pack - everything works perfect.
I have disabled the ENG language and set default BG language, so all users to see the website on BG.
In fact some users can see the web site in ENG language.

The browser cache/history/cookies are not the reason.
Some PC see the web site in ENG, some in BG with one and the same OpenCart settings.

Please, give me a clue...

Best regards,
omega_bg