Prior art is the technology already in the field at the time the patent was filed.
It can have been publicly disclosed anywhere in the world. It does not have to be in English. It just have to be that people in the field should have been aware of it.
It also has to have been filed before you publicly disclosed it or within one year of you publicly disclosing it.
In almost every other country, there is no one-year grace period.
In a weird scenario, if you are not ready to patent something but need to and do not want to file a provisional patentapplication, you can publicly disclose a patent to ensure that no one else can patent it while giving yourself one year to patent it yourself in the US.