To be able to modify customers, a user needs to have a Project Manager role and have already created a customer. If you do not, we recommend that you read our article that explains how to do so: How to create a customer.
Go to Customers → Customer list → (click on the context menu of a given customer) → Language combinations.
Select Customised language combinations only and click Add source language.
Select source and target languages. Make sure they have been added to the column on the right-hand side. Then click the Add button.
The next time you create a project for this particular customer (see How to create a project for more information), you will only be able to select one of the defined source and target languages.