If you want to change the default language used on the login page, set it as described here.
Procedure in the FM Portal
Path to the starting point: Control Panel - Portal Options - Global Basic Data - Languages
Open the record for the language you want to set as the default language.
Select the Default checkbox.
Click Save in the toolbar.
The next time you access the login page, the default language will be set to that language.
Note: If the configuration parameter "USEREQUESTLANGUAGE" is enabled, the language may differ. Please refer to the notes on this parameter.