First, you will need a base language. Say we'll base the translation on english.
On Admin panel , Go to : Settings => Languages => English
And then click on export
You will have an XML File, edit it with your favourite text editor ( Microsoft Word cannot be used, Notepad , Notepad++ or Sublime is preferred)
When you open the file, start editing and replacing information inside the tags only , as follow:
* The editor in the above photo is Sublime Text
When done with the file, save it back and head back to Settings => Language => Add New Language , and choose the file you just edited
When done, the new language will be added to your list of languages
You can set the language as default be clicking the language, then clicking on "Set as Default"