Support of multiple domains

[alexander-myltsev]

I'm using flask_user with babelex. flask_user provides i18n for its registration pages, but not for emails it's sending. I'd like to implement only translations for my custom emails, but not for the standard pages. I can provide my own domain to Babel instance. I'd like flask_user to get email translations from my custom Domain, but fall back to its own on standard pages. Having two Domains -- mine and flask_user's -- how can I merge them to the single domain and pass it to Babel?