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+.
To switch to another language, you need to open the keyboard selection menu, where every language will be listed as an individual keyboard (named Android keyboard). This menu is in the notification area (just as another notification, present when the keyboard is visible).