In order to get to this screen, follow the steps in Accessing the language settings. There are two different places in Drupal that are language based; the first is the user interface, which includes components like the Admin Toolbar. The second is the language of the content from nodes and other content sources. For our purposes, these two should always be the same, so always use the same settings for the two different lists.
In general, there are only four ways to configure this form, depending on your circumstances:
Your primary URL is uni-lingual (ie English and French users would use the same URL to get to your site).
Check the box for “URL” and make sure the “Use Canonical Domains” and “Use Canonical Paths” are unchecked. This will prepend the text strings “en” and “fr” to all your paths in order to determine their language. If a language is not specified, the default is English.
Your primary URLs are different in English and French and...
… they have the same domain.
Check the box for “Use Canonical Paths” and make sure “URL” is unchecked. The path component will be used for language detection.
… they have the same path (or no path).
Check the box for “Use Canonical Domains” and make sure “URL” is unchecked. The domain will be used for language detection.
… they have different domains and path.
Check both the boxes for “Use Canonical Domains” and “Use Canonical Paths”, ordering Domains to be above Paths”. Make sure “URL” is unchecked. First the domain and then the path will be used for language detection.