Language Support > Setting the Language Environment Variable > Detecting a Value for LANG or PRO_LANG
  
Detecting a Value for LANG or PRO_LANG
When there is no value set for LANG or for PRO_LANG, the application attempts to detect the locale automatically from the settings in the Regional and Language Options dialog box or the Region and Language dialog box (accessible from the Control Panel). However, if the regional language packs are not installed, or the current regional setting is not recognized, then the application defaults to English.
 
• The value you set for LANG or PRO_LANG supersedes the default system locale.
• To run the application and Windows in different languages, you must set a value for LANG or PRO_LANG. For example, you can have Regional Options set to German, and run the application in French.