Stings in the theme (or in a plugin or even in WordPress) must be translated by the default WordPress translation method, this means you have to translate .po and .mo files, from several methods i always use Loco translate plugin which offers translation directly in the wordpress UI,
if some strings are not found by the plugin you have to search them in the file and translate them manually, for this i suggest to use this plugin which search strings in the theme files:
The .po and .mo files acts like a catch all so it can miss some translations and that is why doing the translation manually works a lot better is most cases.
Please let us know if you are still having an issue with this.