first of all big thanks to anyone willing to help, now to my issue:
When I change the wordpress language in general settings to anything but default (cs_CZ in my case) the texts are changed properly but the value of default language stays en_US.
I tried to google the issue, but no luck. Everyone else has issues with changing the language, but I dont - it just refuses to change the default value for some reason but still accepts the texts change.
What happens in general settings: 1. I click on the language version and choose Czech (English was preselected) 2. I click save changes 3. Texts in the wordpress changes to Czech language, but the language version resets to English for some reason.
I tried to play with wpconfig file, I set profile Languages to Czech. I triple checked that I have all Czech files uploaded etc.
I am starting to believe that the initial WP installation must be corrupted for some reason?
Any ideas how to fix this?