The String Table Editor can be launched from Window→Localization Editor→String Table Editor.

Managing Languages with the String Table Editor, you can

Creating A New Language

The Add New Language drop down displays all Specific and Neutral cultures. For example, To select es-ES, select Specific Cultures→es-ES - español (España) - Spanish (Spain).

Deleting A Language

To delete a language, first select the language in the Localized Strings drop down. Then click the delete button.

Creating A Custom Language

To create a custom language, select Custom in the Add New Language drop down.

Enter a unique name and click Add Language