To detect the language of a page, we analyze the page content, the top level domain that your site is hosted on, and the language and region of the documents associated with your site. We also use the http-equiv="Content-Language" tag as an important input. This tag lets you indicate on a web page what language it is should be in, For example:

This indicates that the page is expected to be in English. However, this is not our only input and if this is incorrect, we will still try and make our best judgment on the actual language of the page.

By making sure that the majority of these sources of information consistently indicate the right language for your page, you help ensure that the language detected for your page by Yahoo! Search will be correct.