A possible solution (confirmed working in 17.04 at least):
1. On 17.04 and earlier: System Settings > Language Support
2. On 17.10 and later: Settings > Region & Language > Manage Installed Languages
3. Configure your preferred language
4. Click "Apply System-Wide (Use the same language choices for startup and the login screen.)"
A possible solution (confirmed working in 17.04 at least):
1. On 17.04 and earlier: System Settings > Language Support
2. On 17.10 and later: Settings > Region & Language > Manage Installed Languages
3. Configure your preferred language
4. Click "Apply System-Wide (Use the same language choices for startup and the login screen.)"