Are you in a location where the native language is Spanish? Does this happen only when using the Search textbox in the top right, or when using google.com, or both?

Google can auto-detect based on the location of your ISP.

Try changing the interface language here: http://www.google.com/preferences

If the Google (Firefox) or Yahoo home page is in the wrong language then see:

• Google: http://www.google.com/support/bin/answer.py?answer=873
• Yahoo: http://help.yahoo.com/l/us/yahoo/homepage/troubleshooting/troubleshoot-02.html

I usually use the search box in the top RH corner. My ISP is located in New Zealand. The language set in google.com/preferences is English - I have never changed this. I've just tried the search box in my gmail page - it works ok - English. Then tried the other box - Spanish!

Think I have found the answer. The default search engine was set to Google Customized Web Search. When I changed this to Google, English was the result. The default search engine is the button immediately to the left of the RH search panel.