Crowdin - Localize your product in a seamless way
MJML - The open source framework for responsive emails
POEditor - The translation and localization management platform that's easy to use *and* affordable!
Stripo - Stripo is an all-in-one email design platform. We enable our clients to build emails of any complexity, including interactive AMP emails, really fast and easy.
Lokalise - Localization tool for software developers. Web-based collaborative multi-platform editor, API/CLI, numerous plugins, iOS and Android SDK.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.