Download the Opera installer.
Goto the URL opera://about, take note of the "install" path, and close Opera.
Uninstall Opera (do not choose to delete you data if it asks) and then delete the install folder if it remains.
Launch the installer, click "options", set your language, uncheck "import data from default browser", adjust the other options as you like, and install.
Then, if needed, double-check that your country code is correct in your Local State file. And, in Opera, goto the URL opera://settings/languages and make sure your language has been added and "display Opera in this language" is set for it via the 3 dots.