At Admin Panel -> Settings -> Switches, the "language selector" switch activates a dropdown selector of languages for multilingual sites. This allows visitors to select their native language if it hasn't been autoselected correctly. If the switch is off, members can change their language in their profile but unregistered guests have no way to change it.

The selector will only be displayed if you have two or more languages loaded. It appears on every page since it's in the wrapper template.