Various online platforms now exist to help automate the organization and integration of human-language translation work in open source projects. "Translation work" here means not just the process of translating the software's documentation, but also its run-time user interface, error messages, etc into different languages, so that each user can interact with the software in their preferred language. (See the section called “Translation Manager” for more about this process.)
It is not strictly necessary to use a separate translation platform at all. Your translators could work directly in the project's repository, like any other developer. But because translation is a specialized skill, and translators' methods are basically the same from project to project, the process is quite amenable to being made more efficient through the use of dedicated tools. Web-based translation platforms make it easier for translators to get involved by removing the requirement that a translator (who may have linguistic expertise but not development expertise) be comfortable with the project's development tools, and by providing a working environment that is specially optimized for translation rather than for general code development.
Until 2013, the obvious recommendation for a platform would have been https://transifex.com/, which was both the premier software translation site and was open source software itself. However, its main corporate sponsors switched to a closed, proprietary version in March 2013, and development of the open source version stopped then. Transifex still offers zero-cost service for open source projects, as does a competing proprietary platform called Lokalise. But your translators may prefer to invest their time in learning a fully open source platform, and there are several to choose from: https://weblate.org/, http://zanata.org/, https://translatewiki.net/, and https://translations.launchpad.net/ (and there are probably others I don't know about, so look around and ask in other translation communities).