If I set LANG=de in setup.inf, language is still English after installation (with loading setup.inf of course).
You also have to install the language file. Only if you install a language file identical to the selected setup language, the installer with make it the GUI language. So in addition to
Regarding Updates: I can't find a setting in setup.inf that allows me to disable updates. I can set it during manual installation, but I can't during unattended installation with SCCM. I don't want WinSCP to check for updates itself, I'm doing that manually and am updating the package manually as well.
Again, there's no such setting. The installer can only enable the checking for updates. It cannot disable it. The checking is disabled by default. So on fresh install, if you do not select an
task, the checking will be disabled. But if you ever enable it, there's no way to disable it with the installer. It can be disabled in GUI only.