@nebulousecho this has been requested for quite some time now...
either one can add ecosia as search engine or (what i do), i just use this opera addon which allows me to set whichever website i want as default when opening a new tab.
I know it's not ideal, but currently there's not many other options.
Google doesn't separate search results that much anymore - like they used to do for different language areas. Sure China still get's the treatment. If Opera continues to be the only browser with forced default engine ~ step 1 would add option to choose a country code (ccTLD) from a short list including .com!