How can I report to Drupal what is the translation in a language of a string passed to t()?
You could browse to the Translate interface page
(/admin/config/regional/translate/translate).
You could use a .po file and import it (en/admin/config/regional/translate/import).


You'll have to enable the Locale module for this (it's in core).
More information is available on Translating a site interface to different languages.