If you configured your system as a translation server you can delete an existing language if you have the source for this language attached.

To delete an existing language, go to Administration > System configuration > Languages > Language administration and press the Delete language button. Select the languages you want to delete.

Note, deleted languages can not be restored! You have to reinstall your system to bring them back. Normally it is best to disable a language rather than deleting it! You can only delete languages that you have installed as source. It is not possible to delete languages that are in a bundled jar library.