In order to change the default language, the one displayed by default in the SAP login screen, you need to change it through the preferences. If you simply select another language while logging in, it will be applied to the current session only, and you’ll have to enter it again the next time you log in.
To definitely change your default language, in the SAP Logon window, go to Options…
Find the SAP Logon Options => General options, and there change the Language. Also make sure the Use SAP Logon language as default logon screen is checked.
Apply the change, and restart your SAP Logon as prompted, all active sessions will be closed.
Then, reopen SAP Logon, you should already see the interface in the requested language, and find your SAP server.
The interface is by default in the selected language, and the proposed Logon Language is now the one chosen in Options… menu.
In case the language you’d like to use is not available, check with your system administrator, he might have to install it.