New Requirement For Qt 6.10 And Up: ICU (Patreon)

Many applications using Qt 6.10 and up have new dependencies: icuuc.dll and icu.dll. These DLLs were introduced in Windows 10 1703 and 1903, respectively, and retroactively added to some earlier releases, as well. Their intended purpose: implement ICU (International Components for Unicode), which provides timezone, date and locale data, among other things.

However, those DLLs are not normally available on Windows 8.1 and below. To use them, you will have to copy four separate items from a system running late Windows 10 or preferably, Windows 11:

icu.dll, icuuc.dll and (unused by Qt but may be used by other applications) icuin.dll, from the System32 directory. Copy to the Second System x64 module directory, typically at %LOCALAPPDATA%\SecondSystem\x64.

the ICU directory under Windows\Globalization (or %windir%\Globalization). Copy to the Windows\Globalization directory on the target system. This contains all the data necessary for ICU to work on Windows.

The Second System packages for Windows Vista, 7, 8.0 and 8.1 have been updated with the relevant DLL definitions to use these modules.