Adding language code to the button link

The option to insert the language code into the link simply means that if a visitor is on a translated version of your site and they click the link, the page they go to will have the proper language code so that they can continue browsing your store in the same language.

For example:

Say your store is by default in English, but you also offer to view the site in French and Spanish using subfolders.

You can set up a link in the button to https://mygreatstore.myshopify.com/products/t-shirt

Notice how that link address doesn't have a language code. If you have a customer visiting your store in French, that link would send them to the English page for the t-shirt. 

By inserting the language code into the link, when the button is clicked /fr/ is added in, resulting in 

https://mygreatstore.myshopify.com/fr/products/t-shirt.

This allows the customer to continue browsing in French. If a customer is browsing in Spanish and clicks the button, they will be redirected to the Spanish version of the page.

No language code will be added if the button links to an external site, which is any site that isn't your store.