TYPO3 Core

yes, patch was applied. It was tested with master 0f21eaad.
I think the problem is in typo3/sysext/core/Classes/DataHandling/DataHandler.php:4738

         // If the translated record is a default language record, set it's uid as localization parent of the new record.
         // If translating from any other language, no override is needed; we just can copy the localization parent of
         // the original record (which is pointing to the correspondent default language record) to the new record.
         // In copy / free mode the TransOrigPointer field is always set to 0, as no connection to the localization parent is wanted in that case.
4738: if (($this->useTransOrigPointerField && (int)$row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] === 0)
            || $table === 'pages') {
            $overrideValues[$GLOBALS['TCA'][$Ttable]['ctrl']['transOrigPointerField']] = $uid;
        } elseif (!$this->useTransOrigPointerField) {
            $overrideValues[$GLOBALS['TCA'][$Ttable]['ctrl']['transOrigPointerField']] = 0;
        }

$row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] is in my test 1

is $row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] === 0 necessary at all?

May be this is correct if (($this->useTransOrigPointerField && (int)$row[$GLOBALS['TCA'][$Ttable]['ctrl']['transOrigPointerField']] === 0)

I have tested it and the result was correct.

Another error is that the 'Translate' button is displayed, and then you can translate the element from language A to C.
The language C is set to the language B in the translation mode.
So it should not be possible to translate from language A to C.