How to change single custom post template by custom taxonomy?

What you would do, is have your default single-article.php. This will get called by default because of the WP permalinks and templating system.

At the top of your single-article.php do the following before your get_header() call:

$language = get_the_terms( get_the_ID(), 'language' );

if ( ! is_wp_error( $language) && $language && 'French' == $language[0]->name ) {
    get_template_part( 'single-article-french' );
} else {
    // english template stuff
    // etc.

That way you will call single-article-french.php when the French category is selected.