Finally I found how to make it work.
This is only for wording into class files. Once you have introduced the trans() function, ex:
$this->translator->trans('Only letters and the dot (.) character, followed by a space, are allowed.', [], 'Shop.Forms.Help')
You should edit the adequate xlf file into /app/Resources/translations/default/ folder, and insert manually , example:
<trans-unit id="dd7bf230fde8d4836917806aff6a6b27">
<source>Address</source>
<target>Address</target>
<note>Line: 211</note>
</trans-unit>
An voila, will show up in BO under Core translations.
Perhaps it helps,