[TYPO3-mvc] Property is not persistable and isn't being saved to the database, but why?

[Stephen Bungert]

I managed found what the problem is with my extension. It wasn't the tca 
it was my model property names, at least for 'l10n_parent'.

the variable $row in Backend::persistObject() shows the 'l10n_parent' 
and 'sys_language_uid' fields correctly, and 'l10n_parent' is saved to 
the database, but 'sys_language_uid' isn't.

It seems to be ignored even though it is set in the $row to be updated.