What are some alternatives?
When comparing Crowdin and Zanata, you can also consider the following products
Transifex - Transifex makes it easy to collect, translate and deliver digital content, web and mobile apps in multiple languages. Localization for agile teams.
Weblate - Weblate is a free web-based translation management system.
POEditor - The translation and localization management platform that's easy to use *and* affordable!
Smartling - Website localization & translation
Pootle - Multiple file formats.
OneSky - Full Stack Localization Solution