The agent default language is the language Unbabel will assume the agent to understand and write in.
The Unbabel Admin will be in charge of managing the agent language settings in the Settings Page, under the Agents section. This will display all agents that are or have been authorized, along with their spoken and writing languages. Next to each agent entry, a pencil icon leading to the profile settings will be present, which you can click in order to make any changes to the language setup.
The default language will be listed under the Language column. Each agent will only have one default language.
For the normal use case, the agent language should remain English.