1. Go to the Configuration tab (the cog icon) > Data > Language combinations > Customise. Select Customised language combinations only and click the (+) symbol next to Add source language.

language combinations in the configuration

2. Select source and target languages. Make sure they have been added to the column on the right, then click the Add button.

language combinations

3. The next time you create a project, you can select one of the source languages defined in the Configuration tab.

limited source languages

4. Target languages have been reduced only to those selected on the Configuration tab.

limited target languages