In order to display UpCart's upsells, we have to fetch the products using Shopify's Product API. However, according to Shopify's specifications, we have to use a reference to the product handle that is contained in the product page URL.

For example, we can use their API to fetch a product that is located in the products/red-rain-coat section of the store.

Unfortunately, this creates conflicts if you are using a store theme or third party translation application that also translates the product handles in the page URLs. This means that whenever we try to fetch the products using Shopify's Product API, Shopify is looking for a particular product handle (in one language) that does not exist when the product gets translated.

Here is a real store example where trying to fetch Spanish product handles return an error from Shopify because the product handles on their system are stored in English:

In order to fix this, we suggest keeping the product handles in the product page URLs the same no matter what the language is set to on the store.