The biggest value that FLOSS projects can give back to their contributors is Attribution. Just tell the world who did what.
In particular with the rise of distributed version control systems, having attribution as part of the version control history is key. Also, MPL 2 builds on that, and removed attribution from the license header.
Thus, a good l10n tool for an open source project should create open source l10n code, with changesets that give attribution to the contributors, no matter where they originally made their contribution.
It should be noted that DVCSes only know about Authors, and possible Committers. Roles like Reviewer or Proofreader are probably best stored in the commit messages.
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