The simple answer would be translating content means, you create a language version of the content. If administrator has enabled any language, then you can translate your content into that available language. But duplicating content is simple process of copy pasting the content item(s). It will copy all versions (including language and item versions and it's child items) with new ID.
So, when you copy and paste the item it will copy entire tree (it's child items).
Translating content means you can use any third-party translation service(https://world.episerver.com/add-ons/episerver-languages/configuring-auto-translation/ ) to translate the content item for you or you have to manually translate it.
Duplicate content will give you option to select the parent language to translate so that all the content and structure will be copied from parent language and you can then update the content.
Yes, You can copy the whole website and create a new website by simply copy and paste the content tree node.
More details on how to use the auto-translate function: https://blog.novacare.no/forgotten-gem-in-episerver-cms-auto-translate/
What are the differences between translating content and duplicating content? Is it possible to clone whole website instead of page by page and block by block?