What are some alternatives?
When comparing Zanata and Crowdin, you can also consider the following products
Weblate - Weblate is a free web-based translation management system.
Transifex - Transifex makes it easy to collect, translate and deliver digital content, web and mobile apps in multiple languages. Localization for agile teams.
POEditor - The translation and localization management platform that's easy to use *and* affordable!
Pootle - Multiple file formats.
Lokalise - Localization tool for software developers. Web-based collaborative multi-platform editor, API/CLI, numerous plugins, iOS and Android SDK.
Phrase.com - Translate your software better. Edit and share locale files online. Work with your translators or order professional translations for your localization project.