Use preferred language from browser for registration flows

What does this MR do and why?

Detect marketing site language option and browser language setting, and set the preferred_language cookie base on that if the language is valid (high translation level).

Solves these two issues:

1. #418290 (closed)
2. #382387

How to set up and validate locally

1. Simulate SaaS
2. Visit local GDK /users/sign_up
3. Delete the preferred_language cookie in browser
4. Change browser setting to one of these languages: Deutsch, Francais, Japanese, Simplified-Chinese, Traditional-Chinese
5. Visit sign-up page again, displayed language should correspond to the browser setting.
6. Delete the preferred_language cookie again
7. Visit /-/trial_registrations/new?glm_source=about.gitlab.com/fr-fr/ (if browser setting is already French, change 'fr-fr' to 'de-de' or 'ja-jp')
8. Displayed language on trial registration page should correspond to the url param.

MR acceptance checklist

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