The $plugin_rel_path argument in load_plugin_textdomain() requires a trailing slash

As per: https://codex.wordpress.org/Function_Reference/load_plugin_textdomain

> $plugin_rel_path
> (string) (optional) Relative path to WP_PLUGIN_DIR, **with a trailing slash**. This is the preferred argument to use. It takes precendence over $abs_rel_path