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

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.