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