Both issues are kind of similar, just opposite.

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With FTP protocol, there's no "Auto" mode for IP (it behaves same as IPv4 selection). But the "Auto" mode is the default. Had WinSCP switched to IPv4 automatically, when FTP is selected, the selection would get stuck if you switch protocol. I'll probably keep it the way it is.

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As for the language, you are correct, this should be improved.
I have added this request to the tracker:
Issue 2130 – Separate selection for system language
You can vote for it there.

1. IP version setting "Auto" disabled for FTP sites

In the Login dialog, edit an FTP site (or create a new one, make sure it is FTP), click "Advanced...", go to "Connection".
The Internet protocol version has "Auto" selected by default but grayed out. So I can go from Auto to IPv4 or IPv6, but once at IPv4 or IPv6, I cannot go back to Auto.
This does not cause real problem for me, just noticed it.

2. Cannot specifically set a language that happens to be the system language.

In the Preferences dialog, "Languages". I can select the language, and the system default language is shown in bold. Then if I do select the system default, WinSCP does not remember the language that I selected, it only remembers that it is using the system default. So if I move WinSCP (with its corresponding INI file) to a different computer that has a different system default, the language of WinSCP will change. This could be desired behavior for some people, but it is a problem for people like me, who administer many different computers, some of which have system default languages that I (or the intended user of WinSCP) do not understand.
A better approach will be to have a setting for "use system default" separate from the setting for the specific language.
For me, the workaround is to use an INI file and edit the LocaleSafe entry manually. But this is also problematic for the following reason. After editing the INI file, if I ever happen to open the Languages tab of the Preferences dialog again (even without clicking on a language), then WinSCP will overwrite the INI file and again put LocaleSafe=0, which will make it change languages when moved to another computer.
Another workaround would be to uninstall the offending languages, but some of my users do prefer those languages and I want my WinSCP package to work for everybody.

Both issues are minor, sorry for the bother, but I really like this software, therefore wish it to be more perfect.

WinSCP 5.20.5 portable on Windows 7