Yes, by default English is installed in UNA. But after creation and enabling the own language you may to turn off it by next scheme:
1) go to Studio->Polyglot system app
2) choose there your new language as default, press "Save"
3) return to Studio and go to English application
4) click on "Actions" icon in the top-right corner (before "?" icon) and switch it from "Active".
After this actions users will see in site only new language.
With the best regards, Leonid