Contributor License Agreement
A Contributor License Agreement (CLA) is an agreement between a contributor (that's potentially you) and Revolutionary Games Studio ry (a legal entity supporting Thrive development). Signing the CLA is required before your contributions can be accepted as the CLA grants us the necessary permissions to use your work in Thrive. The reason for this is different for programming and other types of work. Translations are MIT0 licensed meaning translators do not need to sign the CLA.
Forum discussion about this topic: https://forum.revolutionarygamesstudio.com/t/making-a-cla-for-thrive/792
For non-programming work the licensing situation has been really confusing in the past as we have in-theory been CC-SA licensing our asset files (art, sound, etc.) but all of the submitted work has been put in the repository by programmers who didn't make the work, so it is not clear if the artists actually agreed to release their work under that license. So in this situation the CLA clarifies the work of artists regarding licensing and puts the use of the work in Thrive on a solid basis.
Programming work had a more clear situation by using the GPL license meaning that pull requests made by people naturally took on the requirement (as a derivative work) to be GPL licensed. As such there wasn't really a problem regarding this. However, if we want to have Thrive available on Steam or on consoles (and probably phones) the GPL license is completely incompatible for release on many platforms. Due to this we need to have a CLA in place to grant us the permission to differently license Thrive on these platforms where it is required. The source code will also be provided under the GPL license from Github, that isn't going away, the CLA just makes future options possible and clarifies our legal right to sue someone on behalf of a Thrive contributor (without a CLA all the individual contributors would need to sue someone to enforce the GPL license).
As users are required to agree to MIT-0 licensing their translations before they can make translations on Weblate, people who have made just translations don't need to sign the CLA. But if they explicitly want to, they can do so anyway.