I just updated a website from 2.4 to 3.1.2 and installed translateable-module. Most parts are working now, but I get an error on my news/blog page:

Couldn't run query: SELECT DISTINCT count(DISTINCT "SiteTree"."ID") AS "0" FROM "SiteTree" WHERE (ParentID = 9 AND Datum <= '2015-04-28') AND ("SiteTree"."ClassName" IN ('Page','ArticleHolder','ArticlePage','CategoryHolder','HomePage','ProductHolder','SiteMap','ErrorPage','RedirectorPage','VirtualPage')) AND ("SiteTree"."Locale" = 'de_DE') Unknown column 'Datum' in 'where clause' 

My filter function:

function FilteredChildren($filter = 'Datum DESC') {
	return DataObject::get('Page', "ParentID = {$this->ID} AND Datum <= '" . date('Y-m-d') . "'", $filter);
}

I don't know why it says 'Datum' is missing or unknown. My article's page has this column:

private static $db = array(
        'Datum' => 'Date'
    );
function getCMSFields() {
		$fields = parent::getCMSFields();
		$fields->addFieldToTab('Root.Main', $dateField = new DateField('Datum', 'Artikel Datum'), 'Content');
		$dateField->setConfig('showcalendar', true);
		$dateField->setConfig('dateformat', 'dd.MM.YYYY');
		return $fields;
    }

DataObject::get is not used like that anymore.
DataObject::get('Page', ... - you're looking for a Page, not an article. So nope, that field does not exist (refer to the query - it's only looking at the SiteTree table).

What you should have is DataObject::get('Article', ...

But what you REALLY should have is:

public function FilteredChildren($sort = 'Datum DESC'){return ArticlePage::get()->filter([
	'ParentID' => $this->ID,
	'Datum' => date('Y-m-d')
])->sort($sort);}