backend language

You should not need to put any translation file manually. HikaShop generates automatically the menu translation file based on the translations in the hikashop main language file.
I just looked at the current dutch translation file for hikashop and the back end menus are in english, so it explains why it overwrote your file. Please put your menu translations in your hikashop translation file (in the config under the tab languages) and save it to recreate the file automatically. Also, please share your translations with the share button so that we can include them in our version. That way, you won't loose them next time you update your translation file.