Honor preferred language from browser as user enters registration flow from marketing website

Summary

Provide a consistent single language experience as a user travels from the marketing site, through registration, into the GitLab product.

Context

Today, a user who views the marketing website in a non-English language (let's use the example of French) then advances into the free or trial registration flow, will see the registration flow steps in English - not the (presumable) preferred language they had viewed on the marketing site. Once in the product, the user can select their preferred language by navigating to their User Profile > Preferences then scrolling to the Localization section and selecting their preferred language from the dropdown selector.

Problem Statement

Provide a consistent single language experience as a user travels from the marketing site, through registration, into the GitLab product.

Solution Ideation

• Can we leverage (pass) the preferred language from the user's browser as they transition from the marketing site to the first step of registration to inform what language should be presented for the entirety of the registration flow experience?
• To align with current Localization efforts and priorities: French, German, and Japanese are the first non-English languages we would want to prioritize for translation. NOTE: Translation of the registration flows will be done by the Localization team
• The work here would be to pass the marketing site language to inform the registration flow experience.

engineering note It seems we can merely look at the referrer url and if that language doesn't match our currently detected language, then switch to use it. Until user is registered - see #418290 (comment 1475339119)