nxml-default-buffer-file-coding-system

nxml-default-buffer-file-coding-system is a variable defined in `nxml-mode.el'.
Its value is

nil

Documentation:
Default value for `buffer-file-coding-system' for a buffer for a new file.
A value of nil means use the default value of `buffer-file-coding-system' as normal.
A buffer's `buffer-file-coding-system' affects what M-x nxml-insert-xml-declaration inserts.

You can customize this variable.