Refactor user default preferred_language

What does this MR do and why?

In JiHu repository, we want to modify the default language options.

See JiHu MR: gitlab-jh/gitlab!112 (merged)

Because there are lots of hard-code test cases in Upstream, I18n.default_locale configuration is not applied in all test environment.

We only set this value in JiHu specific test cases.

The purpose of this MR is to make the test pass on the JH side:

falls back to english when I18n.default_locale is not an available language

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