Supposedly, Microsoft's Bing plays close attention to that meta tag and in the case of Joomla, the default might be set to GB.
Brett Young from the Bing webmaster support team suggested to the webmaster to change the meta tag to:
<meta http-equiv="content-language" content="en-us">
Brett said specifically, with Joomla, "I suggest adding the following meta tag to your home page header and verify that there are no tags pointing to "en-gb"--especially if you are using the Joomla CMS."
It seems like this is a common issue webmasters run into when using Joomla. So I thought I point it out here. I believe Google pays less attention to the meta tag, so it might not be an issue with Google.
Forum discussion at Bing Community.