This solution was provided by Sgunhous

Excellent solution, though for the wrong problem :-p ... the issue is to change the default search engine (i.e. w/o using a prefix) away from Google.

But thanks anyway, at least this makes using Google a bit more bearable as I'm usually searching in English but until now Opera was so "helpful" to force me to my native language.

in order to change yout Google's localisation, please try this:

1. Close Opera Browser, go to Opera's user profile directory (you can find the path in O-Menu->About Opera->Paths->Profile or opera:about),
2. Open the "Local State" file, and change the "country" and the "country_from_server" values as follows:
   "country": "us",
   "country_from_server": "US",
   or if you would like, you can set them for the UK as follows:
   "country": "gb",
   "country_from_server": "GB",
3. After that, increase the "timestamp" value also (it's important - without this change it will not work), for example:
   from this "timestamp": "1385978822600000000" to this "timestamp": "1485978822600000000",
4. save the changes, close the file, and reopen Opera.

(This solution was provided by Sgunhouse in another thread.)

All the best

Although I use Google for the default search engine, on my work computer they are going through some server in Brazil [even though I am located in the USA], so I get the Portuguese language version of Google when I type "g" plus the search terms in the address bar. I can bookmark an English version of Google and use that instead, but it's not as handy as using "g" plus the search terms in the address bar.

It would be nice if I could edit the default search engine web address manually...

If it's possible in the original, it's imho highly dubious Opera removed it actively and the malware protection reason doesn't seem quite conclusive for me. It rather sounds like a contract with Google, or there at least would be some hidden power user option to change the default search engine (some .js in a protected directory or a shortcut, I suggest UP-UP-DOWN-DOWN-LEFT-LEFT-RIGHT-RIGHT-UP-UP-DOWN-DOWN-LEFT-LEFT-RIGHT-RIGHT).

Sadly, to me this is an indication that opera has moved away from the "users first" spirit to the "take it or leave it" way after being forced against the wall by the competition :-\