For translation of theme or plugin is best to use Poedit. Open theme or plugin which you want to translate and copy the file with .po ending into wp-content/languages folder. If that folder does not exists, create it. Proper format for file is PLUGIN_NAME-LANGUAGES_CODE.po. You can see sample here wp-content/languages/realia-en_EN.po.

When editing the strings make sure that Poedit is automatically compiling it into .mo version. If you are not familiar with .mo files read more here. Without that the strings will be not translated. After translation of all strings checks if you have set proper language in WordPress admin.

All strings are passed through translation functions so everything is perfectly translatable. Everywhere is used function __() to translate the strings so if you are editing the catalogue properties you will be fine with just this function. Sample usage of translation function __( 'My String', 'domain' ).

NOTE: Out there are many other WordPress plugins which can help you to translate theme. If something went wrong and your translation is not working, it is recommended to contact third party providers because plugin is using standard way for strings translation.