How can I change the language in Emacs when using ispell?

I would like to use the ispell-buffer command in Emacs. It uses the English language by default. Is there an easy way to switch to another dictionary (for example, another language)?

The following command proposes a list of installed dictionaries to use:

M-x ispell-change-dictionary

Usually, M-x isp-c-d expands to the above also.

From the file ispell.el you may specify some options for the ispell commands. This happens by adding a section to the end of your file like this:

;; Local Variables:
;; ispell-check-comments: exclusive
;; ispell-local-dictionary: "american"
;; End:

Note the double semicolon marks the start of comments in the current mode. It should probably be changed to reflect the way your file (programming language) introduces comments, like // for Java.

At the end of a LaTeX file you can use:

%%% Local Variables:
%%% ispell-local-dictionary: "british"
%%% End:

that will set the dictionary to be used just for that file.

Use M-x ispell-change-dictionary and hit TAB to see what dictionary are available for you.

Then write the setting of default dictionary in your .emacs, and add a hook to start ispell automatically for you specific mode (if you want).

For instance, start ispell in AUCTeX automatically using British English (by default English dictionary is American English)

(add-hook 'LaTeX-mode-hook 'flyspell-mode) ;start flyspell-mode
(setq ispell-dictionary "british")    ;set the default dictionary
(add-hook 'LaTeX-mode-hook 'ispell)   ;start ispell

If you want to change the language on a per-directory basis, you can add this to a .dir-locals.el file:

(ispell-local-dictionary . "american")

If you have no .dir-locals.el file already, it will look like this:

((nil .
   ((ispell-local-dictionary . "american")))
)

See the emacs wiki page about directory variables for more information.

For convenience (f7) I added the following to my .emacs:

(global-set-key [f7] 'spell-checker)

(require 'ispell)
(require 'flyspell)

(defun spell-checker ()
  "spell checker (on/off) with selectable dictionary"
  (interactive)
  (if flyspell-mode
      (flyspell-mode-off)
    (progn
      (flyspell-mode)
      (ispell-change-dictionary
       (completing-read
        "Use new dictionary (RET for *default*): "
        (and (fboundp 'ispell-valid-dictionary-list)
	     (mapcar 'list (ispell-valid-dictionary-list)))
        nil t))
      )))

BTW: don't forget to install needed dictionaries. E.g. on debian/ubuntu, for the german and english dictionary:

sudo apt install aspell-de aspell-en

Here is some code to remap the C-\ key to automatically toggle between multiple languages and to change the input method to the corresponding language. (derived from this post: https://stackoverflow.com/a/45891514/17936582 )

;; Toggle both distionary and input method with C-\
(let ((languages '("en" "it" "de")))
  (setq ispell-languages-ring (make-ring (length languages)))
  (dolist (elem languages) (ring-insert ispell-languages-ring elem)))
  
(defun ispell-cycle-languages ()
  (interactive)
  (let ((language (ring-ref ispell-languages-ring -1)))
    (ring-insert ispell-languages-ring language)    
    (ispell-change-dictionary language)
    (cond
     ((string-match "it" language) (activate-input-method "italian-postfix"))
     ((string-match "de" language) (activate-input-method "german-postfix"))
     ((string-match "en" language) (deactivate-input-method)))))
(define-key (current-global-map) [remap toggle-input-method] 'ispell-cycle-languages)