Help users set their default language

Background

As per a converation regarding i18n translation quality, we don't want to automatically set the language for the user as there can be edge cases where this is undesirable.

We do, however, want to help users understand that GitLab is translated and is available in their language.

Proposal

• If we see that a user has their locale language (not location) set to a language other than English, display a prompt to help them change their language
• Ideally, this preference is stored on the server, so if they log in on another machine then their language preference is remembered
• We should only add this prompt once more of GitLab's interface is externalised in order to set good expectations of the experience