To select a default language for a partner, the language has to be enabled in your Company Settings.
Click your company name in the right side of the top menu and select Company Settings in the drop-down menu.
- In the right side of your Company Settings, click the button Go To in the Languages sub-section.
- Select among the languages in the drop-down and click Add Language. In the same dialog, you can see the list of active languages. You can choose to set one of the languages as your default language.