It is very easy to translate the different language files in the POS.
Go to you local installation C:\xampp\htdocs\POS_NAME\components\com_pos\language

Here you can create a new folder. In case of spanish language: es_ES

Copy the three translation files form C:\xampp\htdocs\POS_NAME\components\com_pos\language\en_GB and replace EN_en_.... with ES_es_.... in the file names.

Then open the files with notepad and translate the different words.

If there is an update you won't lose your translations.