New featured deployed: If you have invite links to give away (you get those by writing reviews), you can now add a default language by appending ?signupLanguage=<code>, e.g., ?signupLanguage=de for German (see footer for list of supported codes) to the invite link URL.
That way if, e.g., you want to invite #Esperanto speakers, they'll get the Esperanto user interface right from the get-go, without having to click through English. :) Thanks to Patrick Noll for the PR.