Prevent LearnDash Notifications from matching notifications created for a different WPML language translation of the same course.

ldn-wpml-language-filter.php

<?php
add_filter( 'learndash_notifications_are_triggering_objects_valid', 'ldn_require_same_wpml_language_after_compat', 9999, 4 );
function ldn_require_same_wpml_language_after_compat( $valid, $trigger, $notification, $args ) {
    // Only act when the notification was already considered valid.
    if ( ! $valid ) {
        return $valid;
    }

    if ( ! is_object( $notification ) || ! is_array( $args ) ) {
        return $valid;
    }

    // If WPML isn't present, do nothing.
    if ( ! has_filter( 'wpml_post_language_details' ) && ! has_filter( 'wpml_element_language_code' ) ) {
        return $valid;
    }

    $get_lang = function ( $post_id ) {
        $post_id = absint( $post_id );
        if ( $post_id <= 0 ) {
            return '';
        }

        // Try post language details first.
        $details = apply_filters( 'wpml_post_language_details', null, $post_id );
        if ( is_array( $details ) ) {
            if ( ! empty( $details['language_code'] ) ) {
                return (string) $details['language_code'];
            }
            if ( ! empty( $details['language'] ) ) {
                return (string) $details['language'];
            }
            if ( ! empty( $details['code'] ) ) {
                return (string) $details['code'];
            }
        }

        // Fallback to element language code filter.
        $code = apply_filters( 'wpml_element_language_code', '', $post_id );
        if ( is_string( $code ) && $code !== '' ) {
            return $code;
        }

        return '';
    };

    // Determine notification post ID.
    $notif_post_id = 0;
    if ( isset( $notification->post ) && ( is_object( $notification->post ) || is_int( $notification->post ) ) ) {
        $notif_post_id = is_object( $notification->post ) ? ( isset( $notification->post->ID ) ? (int) $notification->post->ID : 0 ) : (int) $notification->post;
    } elseif ( isset( $notification->id ) ) {
        $notif_post_id = (int) $notification->id;
    }

    if ( ! $notif_post_id ) {
        return $valid;
    }

    $notif_lang = $get_lang( $notif_post_id );
    if ( '' === $notif_lang ) {
        // Couldn't determine language; don't interfere.
        return $valid;
    }

    $checks = [
        'group_id'  => 'groups',
        'course_id' => 'sfwd-courses',
        'lesson_id' => 'sfwd-lessons',
        'topic_id'  => 'sfwd-topic',
        'quiz_id'   => 'sfwd-quiz',
    ];

    foreach ( $checks as $arg_key => $post_type ) {
        if ( empty( $args[ $arg_key ] ) ) {
            continue;
        }

        $raw = $args[ $arg_key ];
        if ( ! is_int( $raw ) && ! is_string( $raw ) ) {
            continue;
        }

        $arg_id = (int) $raw;
        if ( $arg_id <= 0 ) {
            continue;
        }

        $arg_lang = $get_lang( $arg_id );
        if ( $arg_lang === '' ) {
            continue;
        }

        // If languages differ, prevent this notification from matching.
        if ( $notif_lang !== $arg_lang ) {
            return false;
        }
    }

    return $valid;
}