Adds a language selector to the Location rules for ACF, allowing you to restrict a field group to appear only on admin pages of the selected language.

acf_wpml_language_rule.php

<?php
/**
 * ACF Rule: adds Post Language type
 *
 * @author @theMikeD
 *
 * @param array $choices, all of the available rule types
 * @return array
 */
function cnmd_acf_rule_type_language( $choices ) {
	$choices['Post']['post_language'] = 'Post Language';
	return $choices;
}
add_filter( 'acf/location/rule_types', 'cnmd_acf_rule_type_language' );



/**
 * ACF Rule: populate language option based on active languages.
 *
 * @param array $choices, available rule values for this type
 * @return array
 */
function cnmd_acf_rule_values_language( $choices ) {

	$active_languages = apply_filters( 'wpml_active_languages', NULL, 'orderby=id&order=desc' );

	foreach ( $active_languages as $code=>$properties ) {
		$choices[$code] = $properties['translated_name'];
	}
	return $choices;
}
add_filter( 'acf/location/rule_values/post_language', 'cnmd_acf_rule_values_language' );



/**
 * ACF Rule: Performs the test to determine if the ACF field group should appear for the language test.
 *
 * @param boolean $match, whether the rule matches (true/false)
 * @param array $rule, the current rule you're matching. Includes 'param', 'operator' and 'value' parameters
 * @param array $options, data about the current edit screen (post_id, page_template...)
 * @return boolean $match
 */
function cnmd_acf_rule_match_language( $match, $rule, $options ) {
	if ( ! $options['post_id'] ) {
		return false;
	}
	$post_type = get_post_type( $options['post_id']);
	if ( 'page' !== $post_type && 'post' !== $post_type && 'resource-item' !== $post_type) {
		return false;
	}

	if ( ! defined( 'ICL_LANGUAGE_CODE' ) ) {
		return false;
	}

	$rule_match = false;
	if ( ICL_LANGUAGE_CODE === $rule['value'] ) {
		$rule_match = true;
	}

	if ( '==' == $rule['operator'] && $rule_match) {
		$match = true;
	} elseif ( '!=' == $rule['operator'] && ! $rule_match ) {
		$match = false;
	}
	return $match;
}
add_filter( 'acf/location/rule_match/post_language', 'cnmd_acf_rule_match_language', 10, 3 );