If you get an error message “select one of the languages” while trying to logon SAP system with specific language, then you should check the related language key with the program RSCPINST.

In the below example, we are getting the “select one of the languages” error while  trying to logon with “italian” language ;

If we are sure that, italian language is installed, then we need to check “IT” key via report RSCPINST

In this case;

SE38 –> RSCPINST –> F8

In our case, italian language is installed but “IT” key is missing in the below screen, then we need to add “IT” key in the related field;

Add –> “IT” 

Finally save the change with “Activate” button

Now you can try again to logon with the related language which you already tried.