ConveyThis app for BigCommerce has a powerful language editor to manually proofread and change the machine translations that you obtain automatically after activating the app.
You have a choice of two options: manual side-by-side translation, or visual editing interface.
Both are available for your free of charge and have slightly different options. If the text editor allows you to edit languages in the text only way, the visual interface allows to load your BigCommerce site in the web browser and edit translations anywhere on a page. Our users indicated that visual interface is more preferable as it allows to guess the correct context for the translated sentences and pick better translations.