Use source files to make translations on native language, via program POedit.
Download the latest version of Poedit.
You need to create new translation using PO file or POT template.
Select the file with the language you are going to do the translation. Pay attantion to select file PO Translation File (*.po).
Select the language of the translation.
Now you can start to translate you theme.
Don't forget to save all changes as new file.
Go to Settings > General > Site Language > select the required language and you can see the results of translation on your website.