Solaris 11.4: Mount a Windows Server wont show Umlaut correct in a "LANG=de_DE.ISO8859-15" env

We use Solaris OS 11.4. Our local environment is a de_DE.ISO8859-15 - we do not use UTF-8 at all.

We try to mount a Windows Share view smbfs which works fine, but some characters are shown incorrect.

Instead of 

Stör_04.07.2023.pdf*

we see

Stör_04.07.2023.pdf*

If I would use a UTF-8 Console it would be shown correct. As our programs cannot work with UTF-8 we have to use ISO-8859-15

Here a example:

mount -F smbfs -o user=myuser,domain=mdomain,fileperms=0777,dirperms=0777 "//fileserver/remotepath$" "/mnt/localpath/"

Normally I would use option iocharset=iso8859-15, but this option seems not to be available in Solaris.