flyspell-default-dictionary

flyspell-default-dictionary is a variable defined in `flyspell.el'.
Its value is

nil

Documentation:
A string that is the name of the default dictionary.
This is passed to the `ispell-change-dictionary' when flyspell is started.
If the variable `ispell-local-dictionary' or `ispell-dictionary' is non-nil
when flyspell is started, the value of that variable is used instead
of `flyspell-default-dictionary' to select the default dictionary.
Otherwise, if `flyspell-default-dictionary' is nil, it means to use
Ispell's ultimate default dictionary.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.