anyone please?

What is the value of the pref general.useragent.locale on the about:config page?

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.

en-US

Maybe I didn't explain it clear. I'm using english version of Firefox, everything is set to english, all extensions were in english as well, but yesterday some of them have changed to czech. I'm living in Czechia, but have never set any czech settings in Firefox, so I'm really confused what could have caused this.

Well, I'm honestly disappointed by Firefox dev's reactions = none!

Same thing happened to me today and I found out it's because I installed an extension from downloaded file. So check the Firefox bug somewhere in the extensions installer.

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Sorry, but I don't wanna spend any more minute by trying this, cause it happened on two separate computers same way I described above. It seems to be a Firefox bug for me.