To install a Drupal 7 module translation, this is, a .po file (for instance, the fullcalendar spanish translation) go to Administration » Configuration » Regional and language (/admin/config/regional/translate).
Click the ‘Import’ tab and use the ‘Import translation’ form.
What if there are still some strings translations missing? No worries, just go to /admin/config/regional/translate/translate and add them (by hand).
Tip: use Localization update module 