Locale Determination for Submit Interest Confirmation Emails

For campaigns that send a confirmation email upon signup, we're defaulting to sending the email in the campaign's primary language. It would be valuable to change this to a more intelligent system. The most obvious solution that comes to mind is storing the user's browser locale along with their interest submission, but there may be a more elegant approach.

Start by tracing the interest submission flow and the confirmation email locale selection. Compare the current campaign-primary-language behavior with the proposed browser-locale approach, then establish an agreed design and acceptance criteria for choosing the email language.

Written by the indexing model from the issue text.