Parcelous Order Tracking

Thank you for your review.
Unfortunately, the issue you're talking about is on the Shopify's end.
We display the tracking page at your store via the so-called "app proxy" feature, you may read more about it here: https://shopify.dev/tutorials/display-data-on-an-online-store-with-an-application-proxy-app-extension
This allows only one strict web-address for such a page, which excludes the possibility for it to work with a language subdirectory, as you've said. We hope that Shopify will fix this in the nearest future. There's nothing we or any other developer can do here.