Problem solved, or at least one problem was solved 
The line code you mentioned does not exist:
$langstrings = $this->_loadStringsByTemplate($template, $members, $members_data, $lang_files, $replacements);
is actually:
$langstrings = $this->_loadStringsByTemplate($template, $keys, $members, $members_data, $lang_files, $replacements);
so it was enough to invert those 2 lines like you suggested and the notification for a liked message arrives correctly.
Now if someone @mention me I get this message by mail:
radu,
la presente email รจ per avvisarti chea ha appena citato un tuo messaggio in https://www.skodaclub.it/index.php?msg=188466 su Skoda Club Italia.
Saluti,
Il team di Skoda Club Italia.
My nick is correct, but is missing the author of the mention.