First of all, for all intents and purposes, Kryptonians are humans. They don't have superhuman powers on Krypton; they have superhuman powers on Earth because their biology gets altered in Earth's environment. If anything, Earth makes them alien.
This would be like Perry White going to Krypton and discovering that he can fly. Perry White's a human, yet Krypton's environment alters his DNA. Perry White is now an alien.
If I had to make a series of ridiculous assumptions, I would say that Krypton takes place in a parallel reality (one out of an infinite number of parallel realities), where they are human and speak English just like on Earth. Kal-El's ship is sent through a rip in the fabric of space-time, sending him to an alternate universe where Earth is.
18,000 years ago, when a group of Kryptonians crash landed in the Canadian Arctic, Krypton retrieved data gathered from the ship on Earth, and learned over time through research that the humans of Earth spoke many languages including English and emulated it (along with all other languages) for their own communications.
bad place, I can go all day.
Kryptonians are humans in the deep future. After Earth's resources are used up, humans are sent to Krypton to inhabit the planet and to continue the human species. Once Krypton is discovered to also be doomed like Earth (their former planet) was, Jor-El sends Kal-El back in time in a time-machine to the planet where the Kryptonian species (human beings) originated: Earth. So Superman is a human being. Bam.
See? Plenty of ideas. Choose your own adventure.