To change the language of the sellacious backend, first, install the language other than default language (English). Then on right top of the sellacious backend, you can switch the language of the backend.
As for the frontend, install the language you want while installing Joomla administrator..
Or you can also after you have installed the Joomla already. Install the language you want. Make sure they are published in "Content Languages", otherwise they will not appear on language selections. Publish Joomla's "Language Switcher" module. Enable the "System - Language Filter" plugin. To more know about it follow the below link.
I hope this helps.