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The interface text (like the "Log in" button and the "Add new comment" text) is in English by default, but can be translated. For many languages, there are completed or partly completed translations available. (See the locale module on how to use them.)
All languages need more translation contributions. Some have only incomplete versions of the text in core, so that parts of the interface will show up in English. Others may be complete but need corrections and improvements of the language. And no language has a complete set of translations for all contributed modules.