When installing ECTR there are always all supported languages installed.
A list of supported languages can be found on Product Availability Matrix → Display all Product Versions and search for "ECTR" → open the details for the Product Version and goto Tab "Technical Release Information".
Within SAPlogon.txt you can define a list of possible logon languages with property plm.logon.gui.languages.
Path: %INSTDIR%\customize\config\
For the correct abbreviation you can look into ectr.txt.
Path: %INSTDIR%\basis\dictionary\en
