I can give you step by step instructions in English at least.
There's a utility that can read your password file for you and show you your usernames and passwords. Even though Opera is having trouble reading its own password file, the utility might have better luck.
Here's how you try it:
In Windows, click the start button, goto "Settings -> Privacy & Security -> Windows Security" and click "Virus & threat protection".
Under "Virus & threat protection settings", click "Manage Settings".
Then, turn "Real-time protection" off. You'll turn it back on when you're done.
(The reason you need to turn it off is that if you don't, Microsoft will flag the utility as a potentially-unwanted program, remove the utility on you and pop up a warning that looks scary like a virus warning. The utility is legit though. Note though, if you have a different anti-virus, you'll have to go in its settings and disable its protection for a bit.)
Then, goto https://www.nirsoft.net/utils/chromepass.html and scroll way down the page till you see the "Download ChromePass" link and the password for it (write it down or save it somewhere) below the link. Then, click the link to download chromepass.zip to your downloads folder.
When the download is finished, close Opera if it's open, open up Windows Explorer (File Explorer), go into your Downloads folder, right-click on chromepass.zip and choose "Extract all" and type in the password for the zip file (that's listed below the download link on the page) when asked.
Once it's extracted, you should be taken to the extracted "chromepass" folder. If not, in your Downloads folder, double-left-click on the "chromepass" folder to go into it.
Then, once you're in the folder, you should see ChromePass.exe. Double-left-click it to load it.
When ChromePass loads up, it should automatically show you all your login entries from Opera (and Chrome). For the Origin URL column, you'll see what site an entry is for. Then, you just need to look at the User Name and Password columns to see your username and password.
If everything is there, you can hit ctrl + a to select all entries and then hit ctrl + s to save them to a txt file so you have them.
Then, in Windows Explorer (File Explorer), with "File name extensions" and "Hidden items" enabled under "View -> Show", go in to the "C:\Users\yourusername\AppData\Roaming\Opera Software\Opera Stable\Default" folder and delete the "Login Data" file since Opera can't read it properly. (Or, befor you delete the Login Data file, you can try the alternative method mentioned later.)
Then, you can start Opera and when you need to log in to a site, type your username and password for the site, log in and choose to save it in Opera. Or, you can goto the URL opera://password-manager/passwords and premptively add all your login entries.
Now, if ChromePass doesn't show your usernames and passwords, your "Login Data" file is probably damaged where you won't be able to retrieve your passwords from it. If that's the case, you can try the alternative method below. If not, you'll just have to delete the Login Data file and start with no passwords and not know what they are unless you have them written down somewhere or you have them memorized.
Once you're all done, you can go back and enable real-time protection.
As an alternative, and a more-advanced method, see https://forums.opera.com/post/398128 where you can download and try my Python script that can extract your usernames and passwords from your Login Data file and export them to a password CSV file that you can import into Opera at the URL opera://password-manager/settings. But, you'll need to be able to follow the instructions in the zip file and know how to install Python and run the Python script.
