I have an Xperia (SOL21).
How can I fast switch to another keyboard language, using the default keyboard?
I'm mostly typing in Japanese, but usually swiftly change to Swedish and English (on a non-Android device). How do I do it on Android?
I would recommend leaving the stock keyboard and using the Google Keyboard. It does have a language change key that is great for changing inputs, unlike some stock keyboards. I've set up English (QWERTY), English (DVORAK), and Spanish. Its compatible with Android 4.0+.