where to add this filter? in which file should be added?

Question

For wordpress language switcher, this post was helpful for others, but I don't know where to add the mentioned filter

How to change language file used by _e function

add_filter('locale', function($locale) {
    return esc_attr($_GET['language']);
});

Answer 1

Add this code to your functions.php file in your WordPress theme files.

See this article for some additional help: https://www.rosehosting.com/blog/how-to-add-code-to-functions-php-in-wordpress/