I have Ubuntu 12.04, and firefox 13. When I do google search in firefox, it changes from default English to local language. I can't find google.xml to change the language settings, and the google's language search settings don't hold the saved default language
To Change the language to English you'll have to do some work. Here's how.
The goal is to change Firefox default query string for Google searches.
Quoted from this site:
Making it stick
Each time you'll update Firefox, you will have to re-apply your
changes to the XML file. So it is generally easier to just duplicate
the google.xml file as for instance google_en.xml. In the new XML
file, add the hl parameter, and change the name of the search engine.
Then, in Firefox, select your new search engine. Being a custom search
engine, it will not be erased by an update of Firefox.
