For 14.1 Open a support case and request the fix T51R184 for 17.0 or newer the fix was included.
Add to the portal-ext.properties file the line bellow:
Where xx correspond to the initials of the correspondent language, as an example bellow it will set the default language to spanish to the new users.
Then change default language in USS control panel:
1) Open a browser
2) enter site > http://<USS-servername>:port/group/control_panel
3) Login with administrator account (example: CASMAdmin)
4) On the left Panel >>Go to >> Portal >> Portal Settings.
5) on the Right panel under Unified Self-Service box, Under Miscellaneous click on " Display Settings"
6) Under Language and Time Zone select the default Language desired and click on Save Button.
Next new created user will see the portal with this Language as default.