In Firefox
Use about:config
(There is a typo in qps-ploc in the screenshots, but you surely get the idea)
In Chrome
(Edit: refer to the comments for ways to avoid Chrome override the setting in new versions.)
Edit the file C:\Users\USERNAME\AppData\Local\Google\Chrome\User Data\Default\Preferences
, and add:
{ ...
"intl": {
"accept_languages": "qps-ploc,en-us,en"
},
...
}
For example: