Use case diagram is "A list of steps, typically defining interactions role and a system, to achieve a goal." (according to http://en.wikipedia.org/wiki/Use_case).
Login should not be included in use-case diagram, according to http://discuss.joelonsoftware.com/default.asp?design.4.7340.13: "Logging in is just a by-product of other use cases"
How about language switching on a web page?