How to translate new-added text?
You should never change code in Drupal core, change back to the original code in contact.module first.

When you have the Locale module enabled and have added your language (Configuration -> Regional and language -> Languages) you can translate strings. You will find the translate interface under Configuration -> Regional and language -> Translate interface.

Search for the English phrase first to find the string you want to translate (use exact spelling, it's case sensitive) and add your translation.