How do I change the language for the web portal and audio voice prompts?

You simply need to change the language of the user. This can done globally (for all users in the account) or on a per-user basis. 

Globally

To change the language of all account users globally:

1. Go to Control Center > Account > Timezone & Language

2. Edit the Language field

For a single user

It only edit for a single user

1. Go to Smart PBX > Users, and click the row of the specific user to edit

4. Find the icon that looks like a flag, and choose your language

Note: Single user edits always override global settings. 

In addition to updating the language of the web portal, this will also change the language of user voice prompts (e.g. diailing *98 for voicemail). If the change is done globally (via the Control Center method), then the language of all prompts will change.