Hi,
I've had the same issue and searched sometime to solve this.
It's documented at http://doc.prestasho...+PrestaShop+1.5 paragraph 'E-mail template translations'
To translate the subject headers of mail messages generated by /controllers/front/ContactController.php look into the file /themes/name_of_the_theme/mails/iso_code/lang.php
This file contains translations of the mail subjects.
For the dutch language I've added the following lines to the file /themes/name_of_the_theme/mails/nl/lang.php to translate the ContactController.php generated e-mail messages:
$_LANGMAIL['Your message has been correctly sent #ct%1$s #tc%2$s'] = 'Uw bericht is verstuurd #ct%1$s #tc%2$s';
$_LANGMAIL['Your message has been correctly sent'] = 'Uw bericht is verstuurd';
!! BE AWARE !!
Always be aware to close each line with a semicolon ;. If you forget this tiny character, the complete site will fail to load when you upload the modified translation file!! If you get stuck into this nightmare, get yourself a strong cup of coffee because it might take you all night searching for this little character.
The default translation for in ContactController.php
$_LANGMAIL['Message from contact form\').\' [no_sync]'] = 'Bericht van contactenformulier\'). [no_sync]';
does not seem to work properly. Reduce this translation to the first section only
$_LANGMAIL['Message from contact form'] = 'Bericht van contactformulier';
This line only translates the first section of the subject line and works just fine.
Hope this answer will help someone save some time