I have tested this on at least 3 installations Ubuntu 1604 spanish. If you do the easy install, then open the browser and login as Administrator, then the setup wizard starts and asks for language. There I enter spanish (spanish-ar also tested), the wizard starts good, but seems that has some problems finding items because its translations defers from the “original” english entry.
For i.e., when you reach the point where products might be created… you enter a test product, unit, and value, click Create, and a pupup appears telling “Product group not found” (in spanish)
So, I closed the wizard and looked for Product Tree/Groups… there the product child is translated to spanish, so it reads “Productos” (not products).
So, the wizard will only continue/finde the “Products” group if you rename it from “Productos” to “Products”
I think this problem might also happens on other wizard steps… will try to use ERPNext only in english for now, and specially during the installation.
Hope someone can clarify… I dont trust using for production ERPnext in other than English for now…