Translation displayError message

[ThibaultWUN]

In parentOrderController (Front controller) around line 100 in my file, I have some error message

$this->errors[] = Tools::displayError('You must enter a voucher code.');
$this->errors[] = Tools::displayError('The voucher code is invalid.');

My website is in English and French but I cannot find where the translation occurs. In the French version of the website the text is translated but I added a condition in which I have another 

$this->errors[] = Tools::displayError('My error message that needs translating.');

I search in Localization -> Translations but I could'nt find it. Any ideas ?