I agree with the fact that language files would be good. But I would like this idea to be extended to plugins, also.
I mean, both pre-installed modules and user-installed plugins would use the same model of language files. This way, translation would be even easier, not to mention the possibility of doing a built-in translation manager.
The translation manager would let the user: edit language files; create language files; save the language files (so it can be shared); select main language file; permit readers to chose which language they want the site to be displayed(*).
(*) By that, I do not refer to content translation, but to system translation. I mean the user would continue to see the content in the main language if a translation isn't provided yet, but the system (buttons, links, titles, plugins, etc.) are on the reader's language.
But I also stick with the polyglot idea. It would be nice to provide the post author to translate its text to another language easily.