If you have created a Bot and it’s enabled two languages and if the default language is selected as English, when publishing the Bot you will have checkboxes which are already checked with the default language and remaining are unchecked. These checkboxes you can choose at the task level as well. which means task1 is in English and task 2 in any other language. Now you can check and uncheck the required languages. Once you check any language and publish the bot you will not change the language again.
And regarding the other issue that you have reported, When you have enabled a language a copy of the existed bot will be created which is a virtual kind of thing. When you have the copy of the task whatever the changes that you have made those will be affected by the default task as well. This is an expected behavior. Consider if you have 20 nodes in the Bot configuration and it’s having 3 messaging nodes and remaining are logic. If you wanted to change the logic, you need not change in both of them