Thank you for helping me Lefteris!
The site is not multilingual.The site is in Bulgarian.
I just gave an example with latin tags.

Do you use any extension for multilingual functionality? Or just the core multilingual features?

Hi,Lefteris!
I do not use any extension for multilingual functionality.

Are the tags published? Adding the language variable to the URL gives back an error that the tag cannot be found www.hisarlaka.com/bg/component/k2/itemlist/tag/diet . I am running out of ideas.

Hi,Lefteris!Tags are published but not working. It give an error 500.
I do not know how to go on with K2...I wonder what to do to fix the error.

Try to disable all the third-party installed system, K2 and content plugins on your site one by one until you find what is causing this. I recommend to do this on a development copy of your site. I also recommend to try to copy your site to a different server ( even a local one on your computer ) so we are sure that this is not caused due to server setup.

Finally try to edit your current .htaccess file and copy into it the contents of the htaccess.txt file found in your site root directory.