On the product page, by standard, it says "SKU" there followed by the item number. Where is that translation? I can not find it.

What base theme & version are you using?

All translations are in the relevant template or view file. Alternatively, full dictionaries are available in the relevant application folder, eg /Advanced/ShoppingApplication/Languages. Finally, if you're on the frontend of the site then you can look them up using

SC.Translations

, eg,

SC.Translations['SKU:']