Bug 892696 - Change default language for /newsletter

On /newsletter and footer email forms, if the user's locale is not
for a language that our newsletters support, then instead of
defaulting the language field to 'en', default to having it not
set, to force the user to choose one of the supported languages
when subscribing instead of just clicking through and maybe not
wanting English but getting it anyway.
Merge pull request #1077 from dpoirier/bug-892696-change-newsletter-language-default