...This obviously applies to writing as well, and most Windows users already know that they have the possibility to easily switch their keyboard layout. For Windows 8 and 10, the shortcut is Win + Space, while those who still use Windows 7 have to press Left Alt + Shift.
The language you are currently using is usually displayed in at the right side of your taskbar. But if, for any reason, you need an extra indicator, you could try Layout Indicator.