Beware, however, that if you change ContentObjectLanguage you will not be able to edit any objects that does not have a translation in your new language already.
As David said, you need to update the database. Example (to change to french):
UPDATE ezcontentobject_name
SET
content_translation='fre-FR',
real_translation='fre-FR'
WHERE
content_translation='eng-GB' OR
real_translation='eng-GB';
UPDATE ezcontentobject_attribute
SET
language_code='fre-FR'
WHERE
language_code='eng-GB';
Here's what worked for me, mysql 4.1, also making sure that untranslated objects retain the correct 'real_translation'...
UPDATE `ezcontentobject_name`
SET `content_translation`='eng-CA'
WHERE `content_translation`='eng-GB';
UPDATE `ezcontentobject_name`
SET `real_translation`='eng-CA'
WHERE `real_translation`='eng-GB';
UPDATE `ezcontentobject_attribute`
SET `language_code`='eng-CA'
WHERE `language_code`='eng-GB';