Gargantia on the Verdurous Planet: The Complete Series

While fighting an intense inter-galactic war, a mecha pilot was accidentally warped into a space-time neither he nor the computer of his mecha could recognize. After waking up from a long-time hibernation, he found himself trapped on a planet, with human residents talking in an unknown form of language, using inferior technologies, and - most shocking to him - naturally breathable air.