The translation file you can find in languages folder (/wp-content/themes/willowpillow/languages). Edit the .pot file using POEdit, use the translation field to make replacements. Read here how to do it right.
From the file menu, save file with your language name e.g de_DE.po into languages folder. It will generate both a *.po and *.mo file for your translation.
Next edit wp-config.php located in the root folder of wordpress and define the WPLANG prefix with your language name which should be similar to the translation file name.