From your Online Store dashboard,

1. Click Catalog.

2. Next, click Products.

3. Select Edit Product in the drop-down to right of the product.

4. Under General, scroll down to the "Translations" section to add.

5. For "Product Options", click Options and click +Add New Option.

6. Provide a title and price modifier

7. Click Save and Refresh the page.

Note: if you do not have languages enabled this will not work. To add languages to your store click here