flyspell-babel.el – Switch flyspell language according to LaTeX Babel commands
From the commentary:
Flyspell is an Emacs package that highlights misspelled words as you type; Babel is the standard mechanism for switching languages in LaTeX. There are a number of Emacs packages available that will try to guess the current language of a buffer or part of a buffer, and make flyspell switch to a different dictionary; but I didn’t find one that used the explicit language-switching commands available in a LaTeX file for this purpose. This file makes flyspell use the correct dictionary for the language used in each part of a LaTeX file. Version 2.0 is much faster than 1.0.