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I don't know that you can do it without an extension, however Mozilla has put out an extension called "Firefox Translations" that translates webpages locally on your computer with a downloaded dictionary. So far it works well in my experience, though I don't know any other languages, so I can't really speak to its accuracy. That being said, so far I haven't come across any nonsensical translations.
My cats both like to be near the space heater when its on. So far we haven’t had any trouble with it, but we always stay in the same room and keep an eye on them.