I would like to let you know that after two weeks of investigation and many failures I understood K2 messup the whole language tag system in Joomla.
If you have a multilingual website, and install K2, it will change the website to only each time get the default language tag.

since I use a rtl language on my website, I went through a lot to understand what is causing this error. And it is the new version 2.10.3 .

as soon as you install K2 this happens.
What could be the reason?
Anyone has any solution?

as you can see in the image. the page is in the fa-ir language but the html tag says it is en-gb and this is bad since it changes the whole rtl styles to ltr. and creates a mess.

Here is a quick 60sec video on the problem:
https://i.imgur.com/r4kSlek.mp4

I understood this file is needed on my home page :
K2HelperUtilities
which is in this directory:
..../components/com_k2/helpers/utilities.php

I don't know why my first page which has nothing to do with k2 right now is needing this file.

I would seriously need help.

found the cuplrit:
.../plugins/system/k2

the k2 system plugin mess up Joomla language.

I replaced that file with an older version 2.8 and it is working alright now.
Hopefully we will find what is going on with the new version.

Thanks Fotis,

It was the weirdest problem I ever had. but it arised after I got hacked. I had to install a new fresh joomla and go into the pain of transfering all data to this new one and now I have the latest K2 and it works fine.