Resolved
Overview of the issue
When you update to WordPress 5.3 and you have your debug enabled, you might see a warning visiting Plugins -> Add new -> Commercial page.
This is happening because of changes in WordPress for the function WP_Upgrader_Skin::feedback
Workaround
We are working on fixing this, but in the meantime you can patch it by going to
wp-content/plugins/sitepress-multilingual-cms/vendor/otgs/installer/includes/class-installer-upgrader-skins.php
There change the value:
function feedback( $string ) {
to
function feedback( $string, ...$args ) {