Answer
Use a Report Rule with the 'Preselect Property' action type to set the language property to your desired default — this pre-fills the language selection without preventing the user from changing it if needed. 'Set Property' and 'Set Default Property' action types can also achieve a similar result in most scenarios, but behave slightly differently: Set Property locks the value so the user can't change it, while Preselect Property and Set Default Property still allow the user to override the pre-filled default if they need to print in a different language for a specific run.