ispell-local-dictionary

ispell-local-dictionary is a variable defined in `ispell.el'.
Its value is

nil

• Automatically becomes buffer-local when set.
• This variable is safe as a file local variable if its value
  satisfies the predicate `string-or-null-p'.

Documentation:
If non-nil, the dictionary to be used for Ispell commands in this buffer.
The value must be a string dictionary name,
or nil, which means use the global setting in `ispell-dictionary'.
Dictionary names are defined in `ispell-local-dictionary-alist'
and `ispell-dictionary-alist'.

Setting `ispell-local-dictionary' to a value has the same effect as
calling M-x ispell-change-dictionary with that value. This variable
is automatically set when defined in the file with either
`ispell-dictionary-keyword' or the Local Variable syntax.

You can customize this variable.