Bug: Undefined array key “old”

Error: Warning: Undefined array key "old" in /var/www/html/wp-content/plugins/wp-multilang/includes/class-wpm-taxonomies.php on line 289

Testing Instructions: The problem is that update_description can also be called when the insert path set $this->description (without the 'old' key). This happens when I create a new term programmatically and WordPress fires both the insert and update hooks.

Conclusion: Add a null coalescing for this array key old:

$this->description['old'] ?? array()