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

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 https://github.com/graasp/admin/pull/113#discussion_r2686448611_

Start by locating the email template rendering and building entry points mentioned in the issue, then read how the current language configuration is scoped. The issue proposes isolating rendering in its own process to prevent configuration leaking between operations. Done means successive email renders do not inherit another operation's language configuration.

Written by the indexing model from the issue text.