Extbase Model: setSysLanguageUid not working

I created an extbase model and try to set the sys_language_uid field when creating a new item. For some reason though, it is completely ignored and always set to 0, even when the value I try to enter is definitely 1.

My Model looks like this:

class Ad extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {

    /**
     * sysLanguageUid
     *
     * @var integer
     */
    protected $sysLanguageUid;


    /**
     * @return int
     */
    public function getSysLanguageUid()
    {
        return $this->sysLanguageUid;
    }

    /**
     * @param int $sysLanguageUid
     */
    public function setSysLanguageUid($sysLanguageUid)
    {
        $this->sysLanguageUid = $sysLanguageUid;
    }

    // ... etc. 
}

and in my controller all I try to do is this:

$ad = new Ad(); 
$ad->setSysLanguageUid($GLOBALS['TSFE']->sys_language_uid);
$ad->setSomeOtherParam('xxx');
$this->adRepository->add($ad);

The other param is saved just fine. sys_language_uid exists in ext_tables.sql and in the TCA:

'columns' => array(
    'sys_language_uid' => array(
        'exclude' => 1,
        'label' => 'LLL:EXT:lang/locallang_general.xml:LGL.language',
        'config' => array(
            'type' => 'select',
            'foreign_table' => 'sys_language',
            'foreign_table_where' => 'ORDER BY sys_language.title',
            'items' => array(
                array('LLL:EXT:lang/locallang_general.xml:LGL.allLanguages', -1),
                array('LLL:EXT:lang/locallang_general.xml:LGL.default_value', 0)
            ),
        ),
    ),  
    'some_other_field' => ....
)

Why is setSysLanguageUid not working? Any hints?

it seems like setSysLanguageUid doesn't work because typo3 is UNNECESSARILY CONVOLUTED AND DIFFICULT. For some reason it is possible to set the language ID but only by defining and using these getters and setters:

/**
 * _languageUid
 * @var int
 */
protected $_languageUid;

/**
 * @param int $_languageUid
 * @return void
 */
public function set_languageUid($_languageUid) {
    $this->_languageUid = $_languageUid;
}

/**
 * @return int
 */
public function get_languageUid() {
    return $this->_languageUid;
}

I swear, typo3 is gonna give me a heart attack one of these days....