Isolate email template rendering in own process to not leak i18n configuration

[spaenleh]

I don't think relying on a scoped language to generate or build translated texts for email is 100% reliable. It means you always need to set the language before rendering or building an email and never rely on the default language.
A previous operation might leak on other email/language-related in the future and it would be difficult to catch.

Originally posted by @pyphilia in #113 (comment)