Beside of some issue with the processing of the LCU within New-OSBuildMultiLang, I also recognized, that Default system UI language is not changed for Windows 11.
This is due some change in the behaviour of setting the default system UI language for Windows via DISM (https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/dism-languages-and-international-servicing-command-line-options?view=windows-11#set-uilang)
Unfortunately, this is also affecting Set-AllIntl and it seems like MS doesn't want us to have MUI images any longer.
I have tried to find some quick workaround, but not yet successfully - my fear is, that there is no solution except completely rethinking the usage of MUI images. But first I will focus on processing LCU for Windows 11 to work similar like in New-OSBuild.