Yes, go to Settings -> General -> Language -> Choose your preferred language for displaying pages -> press the Choose… button.

The thing is i already have put latvian there and not russian, but still some pages show russian

Try to put English there on a 2nd place.

have already did that, 1st language latvian and 2nd english.. cant seem to find any place with russian language at my settings

Language may be decided based on the location of your IP address. Check IP location and try using a VPN or proxy located elsewhere.

my ip address is in Latvias capital, Riga, and not any russian ip, so thats not the case

Does it still happen with default settings in a new browser profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

thanks! at developer edition it shows how it should. what should i do now?

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be careful. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

If you want to fix your profile instead of simply refreshing it, you will need to find the issue through a process of elimination.

Backup your profile folder and try resetting all about:config settings to default by renaming the prefs.js file while Firefox is not running.

Note that you lose extension data if you rename/remove prefs.js, you can copy the extensions.webextensions.uuids line to the new prefs.js to recover extension data.

It would be better to restore from the backup once the issue has been identified.