nxml-prefer-utf-16-to-utf-8-flag

nxml-prefer-utf-16-to-utf-8-flag is a variable defined in `nxml-mode.el'.
Its value is

nil

Documentation:
Non-nil means prefer UTF-16 to UTF-8 when saving a buffer.
This is used only when a buffer does not contain an encoding declaration
and when its current `buffer-file-coding-system' specifies neither UTF-16
nor UTF-8.

You can customize this variable.