Base theme translation
Using the Loco Translate plugin
Go to Plugins>Add new> install and activate the Loco Translate plugin
Go to Loco Translate>Themes> select XStore theme. Press "Add new language". Translate strings and Save.
The translation file xstore.pot is in the Translation Files folder in the theme package. Edit the .po file using POEdit, and use the translation field to make replacements.
From the file menu, save the file with your language name and xstore theme suffix, e.g xstore-de_DE.po. It will generate both a .po and .mo files for your translation. Place these files into the languages /wp-content/languages/themes/ folder.