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

[Stephen Bungert]

I'm trying to create records in a BE module. Everything works, but I 
can't set 'l10n_parent' and 'sys_language_uid' for my new records.

After debugging certain things I discovered that the cause is in 
Backend.php in the function persistObject().

The line causing the problem is:

if (!$dataMap->isPersistableProperty($propertyName) || 
$this->propertyValueIsLazyLoaded($propertyValue)) continue;

The isPersistableProperty() function is not returning true.

Looking at the doumentation for isPersistableProperty() I see that this 
is something set by the TCA, in my tca files for my model 'l10n_parent' 
and 'sys_language_uid' are just as they were when created by the extbase 
kickstarter. What exactly needs doing so that these properties are 
persistable?

The documentation doesn't say, and I'm  a bit stuck. All my other models 
properties are set ok. Just these two are not.

Stephen.