Me again! :-[
It seems the line:
$txt['split'] = 'Split';
...in index.english.php isn't used for the stuff shown in the screenshot? :o I changed it to "Geteilt" (which means "topic is splitted") as part of my translation, which differs from "Teilen" (which is "to split" in english), but nothing happens.
$msgOptions = array(
'subject' => $txt['split'] . ': ' . strtr(Util::htmltrim(Util::htmlspecialchars($subject)), array("\r" => '', "\n" => '', "\t" => '')),
It is used, to create and save the subject, so if you changed it after you split the topic it will not change.
I'm saying this because I see at transifex that is translated as TEILEN, so I'm not sure if you changed it before or after. ;)
I changed it after I splitted the thread. Thanks for the explanation.
No problem. ;D