In recent years, various platforms have arisen 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 the translation 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 because such platforms remove the requirement that a translator (who may have linguistic expertise but not development expertise) be comfortable with the project's development tools, and because they are specially optimized for performing translation rather than for performing 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. Transifex is probably still the premier platform in terms of usage, including among open source projects, but its main corporate sponsors forked it into a closed, proprietary version in March 2013. Transifex still offers free hosting for open source projects, but if you prefer that your translators have a fully open source platform to work in, https://weblate.org/ is one, and it offers a commercial hosted service that is free of charge for open source projects as well. Three other open source platforms are Launchpad Translations https://translations.launchpad.net/, http://zanata.org/, and https://translatewiki.net/.