Please note that the Translation Assistance Plugin support has been shut down due to a number of problems. We still welcome your contributions via pull requests on GitHub.
This page is the landing page for the translation assistance plugin's "contribute" link
When you check the "I contribute my translations to the Jenkins project" link, you acknowledge that your submission will be licensed under the MIT license (the same license that the rest of the Jenkins core uses) and you have the rights to release them under the said license.