Proceed to the application settings:

Tap "Dictionary language" and select the language for your dictionary there.

After you change the product dictionary language or the system language of your device, the existing items in your shopping lists are not translated to the new language. The products which were added, edited or ever used by you in your lists before, stay in the language you used them in; the remaining default items in the previously used language are deleted; and the new dictionary in the newly selected language is added.