Redirect to browser language in WordPress + WPML when the default language is in a subfolder (as the rest) and there is no real homepage.

redirect-to-browser-language-wp-wpml.php

<?php

// redirect to language if no language
add_action( 'after_setup_theme', function () {
	if ( is_admin() ) {
		// we are on admin; do nothing
		return;
	}

	if ( $GLOBALS['pagenow'] === 'wp-login.php' ) {
	    // We're on the login page, do nothing
	    return;
	}
	
	// ignore POST request as redirecting them would do more harm than good.
	if ( $_SERVER['REQUEST_METHOD'] === 'POST' ) {
		// we are processing a POST; do nothing
		return;
	}

	// we cannot use the wpml_active_language filter here because -- like any other 
	// API of WPML -- it defaults to the default language, which we are trying to avoid.
	global $wpml_request_handler;
	$req_lang = $wpml_request_handler->get_requested_lang();
	if ( $req_lang ) {
		// the page requested has a language; do nothing
		return;
	}

	// what should we use as fallback language?
	$default_language = apply_filters( 'wpml_default_language', null );
	if ( ! $default_language ) {
		$default_language = 'en';
	}

	// what language is the user's browser set-up? If the user does not pass a language, use the default.
	if ( ! isset( $_SERVER['HTTP_ACCEPT_LANGUAGE'] ) || ! $_SERVER['HTTP_ACCEPT_LANGUAGE'] ) {
		$_SERVER['HTTP_ACCEPT_LANGUAGE'] = $default_language;
	}

	// what languages are configured?
	$allowed_languages = apply_filters( 'wpml_active_languages', array() );
	$allowed_languages = array_keys( $allowed_languages );

	// test the configured language codes against the requested language
	preg_match( '/^(' . implode( '|', $allowed_languages ) . ')/', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $match );
	// if there is no match, defer to the default language.
	if ( ! $match || ! $match[1] ) {
		$match = array( '', $default_language );
	}

	// prepare the redirect; inject the language code.
	// dev note: $_SERVER['REQUEST_URI'] includes GET-parameters
	$redirect = '/' . $match[1] . '/' . ltrim( $_SERVER['REQUEST_URI'], '/' );

	// redirect the user
	wp_redirect( $redirect, 307 );

	// bye bye.
	exit;
} );