Hi Paul,
There shouldn't be a difference between internal and external calls, as CWMS doesn't differentiate between those.
Can you, please, check the following:
1. Dashboard > Settings > Audio > System Audio Language
2. Check the user profile that you are outdialing to outside (Dashboard > Users > find the user profile > Language) and what is set there.
The only difference in the language played during outdials should be based on the user profile and system setting.
-Dejan