The input language settings are per-application. An application which calls a Windows API in order to change the input locale, can set a different keyboard layout, if it is enabled for the user.
Sometimes it also happens, if you accidentally hit the ALT and LEFT-SHIFT keys together. If you don't want to have this behavior, you could "uninstall" all input languages/cultures you don't need in the Control Panel:
Regional and Language Options > Text Services and Input Languages
EDIT: or just check "Turn off advanced text services".
Yet, I've not found out if it is possible to change the default input language via a shortcut or if it's possible to disable the regular fall back to the default input language when opening a new program, but I've found an alternative!
Googling the problem, I came across the following article written by Androgen Belkin:
http://www.script-coding.info/AutoHotkey/AhkRussianEng.html
He explains that Russians always have a similar problem with their keyboards concerning the Russian and English language. Thus, he created a script that checks and switches on user input, if necessary, the default input language when Windows starts. The script can be found on the above link at the very bottom and has been written for AutoHotKey, a free keyboard macro program. Luckily, I am a devoted AutoHotKey user myself.
Instead of Russian and English, I adapted the script to check for the German and English default input language. It works errorless and is very handy because now I can set my default input language with ease :)
Furthermore, AutoHotKey supports compiling scripts into single executables. I don't want to withhold my script from everybody to whom it is as useful as to me.
Switching default input language [Germany (German) <> English (United States)]:
http://www.megaupload.com/?d=A8HKEV4A
Filename: Switching_default_input_language_DE-EN.zip
Contents: (1) Switching_default_input_language.ahk (AutoHotKey script; for editing or re-compiling the script), (2) Switching_default_input_language.exe (stand-alone executable)
Have fun! :)
Best Answer
Control Panel - Language - Advanced Settings - Let me set a different input for each app window