TYPO3 Core

The getRecordOverlay method allows this when the record is rendered in the frontend:

if ($sys_language_content > 0) {
    // Must be default language or [All], otherwise no overlaying:
    if ($row[$GLOBALS['TCA'][$table]['ctrl']['languageField']] <= 0) {
    ...
    }

If no overlay record is found, then it checks for 0 in the original record and unsets it. So IMHO, records with sys_language_uid -1 should be localizable

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44409

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44409