Hi Mikey,
In order to translate the strings using ‘Loco Translate’ plugin, just the .po, .mo files are required.
Can you please search for the text domain as ‘custom-order-statuses-woocommerce’ under the Loco Translate->Plugins submenu? Select the plugin from the list of plugins, then add the required language. Once the language is selected, the strings of the plugin are available for translation.
I have tested this where the strings from our plugin are listed in the Loco Translate plugin.
Please let me know whether the provided information helped you to achieve your requirement.
Regards,
Komal Maru