Software localization project owners and project adminstrators can add a contributor/translator to specific languages in two ways:
In the Project Page
Go to your project's main page, and you will see next to the translation progress of each language, on the right, three Action buttons. To assign a contributor to a language, click on the first button in the row, Add contributor.
In the Language Page
Click on any of the languages in the project page, and you will be taken to the language page, where contributors do the actual translation work. In each of these pages, you will find a side menu. Here, you can assign contributors by pressing on the button called Add Contributor.