I had a Windows where German was the only installed language, and SPM got stuck after displaying the first "Fetching ..." (see my comment from July 25 and 26 on https://forums.swift.org/t/stochastic-errors-resolving-dependencies-and-or-building-with-feb-18-2021-snapshot/44929). Using Process monitor, I found:
swift.exe: RegQueryValue: BUFFER OVERFLOW: HKCU\Control Panel\Desktop\PreferredUILanguages
swift.exe: RegQueryValue: BUFFER OVERFLOW: HKCU\Control Panel\Desktop\MuiCached\MachinePreferredUILanguages
After adding dding "English (United States)"" as additional language and then setting the system language to English, the SPM then first went further (and displayed another error message about false package version which is also not OK as a message, but maybe that is another issue), but on another ty it again got stuck with the above errors.
An error that was always displayed in the process monitor was:
swift.exe: RegQueryValue: BUFFER TOO SMALL: HKLM\System\CurrentControlSet\Control\Notifications\418A073AA3BC8075
(The Swift package was https://github.com/stefanspringer1/SwiftXMLParserDemo.)