Opera will read your language settings from the System Preferences on first start-up and automatically apply an appropriate language file and set the preferred content language based on those settings. Please note that Opera is not available in all languages, and most websites may not be able to serve content based on your language setting.
If you have downloaded an Opera language file separately, go to the "Details" dialog to apply it. Once you have edited these preferences in Opera, your language settings will no longer be affected by changes to the system preferences.
Language in the opera.com website
If you are viewing our website www.opera.com, you can change the language for the site by using the selector at the bottom, left of the screen.