Add the lang in params when doing request tests.

Problem description: Before we set the I18n.locale to :en in our test env. But in the application controller we also set the language
by the params or browser. So if we run request test in different language
environment, the locale will be set differently. When the environment
is not English, there will be some failures when we try to click links
or check content by words in Carpybara.

Solution: Removing the dependency to the running env. We'd better set
the lang for every request in our test.