This afternoon Opera has been updated to Version: 80.0.4170.63, perhaps in this new version those problems have been solved. Luck.

@fonm Hello. That would be to put it in some configuration file of Opera? Any txt?
If I put opera: // settings in the address bar, the normal graphic menu of Opera appears and there it does not allow to change the language.

I have achieved it by installing opera-stable_79.0.4143.72_amd64 that I had already downloaded to the hard drive for some time and then updating to the latest version. I do not know if there is a reliable page where you can download those older versions, I will have to investigate.
It's a shame because if you don't have it in your language it is quite cumbersome and you usually have to use another browser.

@gabriel75 Hello, after breaking my head a lot, I have managed to change the default language and put it in Spanish. I've done two things and I'm not sure which one has worked.
The first thing I did was install an old version of Opera opera-stable_79.0.4143.72_amd64 and then install libudev0 which was showing up as an unsatisfied dependency. There I was able to put it in Spanish and then I went to the Muon package manager (I imagine that synaptic can work as well) and update Opera to its latest version.
I hope you understand I do not speak English