What are some alternatives?
When comparing Smartling and Transifex, 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!
Localize - Localize is translation software that automatically converts the content of your website or app into multiple languages.
Lokalise - Localization tool for software developers. Web-based collaborative multi-platform editor, API/CLI, numerous plugins, iOS and Android SDK.
OneSky - Full Stack Localization Solution
Pootle - Multiple file formats.