$ cat enc.fsx 
printf "System.Text.Encoding.Default: %O\n" System.Text.Encoding.Default
$ LANG='en_DK.UTF-8' fsharpi enc.fsx 
System.Text.Encoding.Default: System.Text.UTF8Encoding
$ LANG='en_DK.ISO-8859-15' fsharpi enc.fsx 
System.Text.Encoding.Default: I18N.West.ENCiso_8859_15
$ LANG='en_DK.GARBAGE' fsharpi enc.fsx 
System.Text.Encoding.Default: System.Text.ASCIIEncoding