Remove hard-code zh on locale and add qa_enforce_locale_to_en

Depends on Upstream MR:

• Add prepend_mod to PreferredLanguageSwitcher
• Add feature flag qa_enforce_locale_to_en

What does this MR do and why?

• Remove hard-code zh_CN on locale settings #2168 (closed)
• Add ff qa_enforce_locale_to_en to enforce locele to en for E2E test cases #1199 (closed)

Behavior

qa_enforce_locale_to_en default false.

• For login users, use the preferred language in user attribute;
• For first come unlogin users, use the default_preferred_language in ApplicationSettings (if turns on the qa_enforce_locale_to_en, this setting will return en) ;
• For unlogin users who switched preferred language, use the user selected.

cc EM @prajnamas

cc QA @WilsonDuan