[...]
$page = TranslatablePage::isTranslationPage( $title );
if ( $page !== false && $page->getMarkedTag() ) {
	[...]
	return false;
}
return true;

public static function isTranslationPage( Title $title ) {
	$handle = new MessageHandle( $title );
	$key = $handle->getKey();
	$code = $handle->getCode();
	if ( $key === '' || $code === '' ) {
		return false;
	}
	$codes = Language::fetchLanguageNames();
	global $wgTranslateDocumentationLanguageCode;
	unset( $codes[$wgTranslateDocumentationLanguageCode] );
	if ( !isset( $codes[$code] ) ) {
		return false;
	}
	$newtitle = self::changeTitleText( $title, $key );
	if ( !$newtitle ) {
		return false;
	}
	$page = self::newFromTitle( $newtitle );
	if ( $page->getMarkedTag() === false ) {
		return false;
	}
	return $page;
}

public function __construct( Title $title ) {
	$this->title = $title;
}

SELECT
	page_id,
	page_namespace,
	page_title,
	rd_from,
	rd_fragment,
	page_is_redirect
FROM
	(
	SELECT
		pl_from,
		rd_from,
		rd_fragment
	FROM
		`pagelinks`
	LEFT JOIN `redirect` ON
		((rd_from = pl_from)
			AND rd_title = 'Main_Page'
			AND rd_namespace = 0
			AND rd_interwiki = '')
	WHERE
		pl_target_id = 12
		AND pl_from_namespace IN (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 470, 471, 828, 829, 1100, 1101, 1198, 1199)
	ORDER BY
		pl_from_namespace ASC,
		pl_from ASC
	LIMIT 102 ) `temp_backlink_range`
JOIN `page` ON
	((pl_from = page_id))
ORDER BY
	page_namespace ASC,
	page_id ASC
LIMIT 51

+------+-------------+------------+--------+-----------------------------------------------+----------------------------------+---------+-----------------------------+------+---------------------------------+
| id   | select_type | table      | type   | possible_keys                                 | key                              | key_len | ref                         | rows | Extra                           |
+------+-------------+------------+--------+-----------------------------------------------+----------------------------------+---------+-----------------------------+------+---------------------------------+
|    1 | PRIMARY     | <derived2> | ALL    | NULL                                          | NULL                             | NULL    | NULL                        | 2    | Using temporary; Using filesort |
|    1 | PRIMARY     | page       | eq_ref | PRIMARY                                       | PRIMARY                          | 4       | temp_backlink_range.pl_from | 1    |                                 |
|    2 | DERIVED     | pagelinks  | range  | pl_target_id,pl_backlinks_namespace_target_id | pl_backlinks_namespace_target_id | 12      | NULL                        | 39   | Using where; Using index        |
|    2 | DERIVED     | redirect   | eq_ref | PRIMARY,rd_ns_title                           | PRIMARY                          | 4       | metawiki.pagelinks.pl_from  | 1    | Using where                     |
+------+-------------+------------+--------+-----------------------------------------------+----------------------------------+---------+-----------------------------+------+---------------------------------+

SELECT
	page_id,
	page_namespace,
	page_title,
	rd_from,
	rd_fragment,
	page_is_redirect
FROM
	(
	SELECT
		pl_from,
		rd_from,
		rd_fragment
	FROM
		`pagelinks`
	LEFT JOIN `redirect` ON
		((rd_from = pl_from)
			AND rd_title = 'Main_Page'
			AND rd_namespace = 0
			AND rd_interwiki = '')
	WHERE
		pl_target_id = 12
		AND pl_from_namespace IN (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 470, 471, 828, 829, 1100, 1101, 1198, 1199)
	ORDER BY
		pl_from_namespace ASC,
		pl_from ASC
	LIMIT 102 ) `temp_backlink_range`
JOIN `page` ON
	((pl_from = page_id))
LEFT JOIN `page_props` `translate_pp` ON
	((translate_pp.pp_page = page_id)
		AND translate_pp.pp_propname = 'translate-is-translation')
WHERE
	translate_pp.pp_value IS NULL
ORDER BY
	page_namespace ASC,
	page_id ASC
LIMIT 51

+------+-------------+--------------+--------+---------------------------------------------------+----------------------------------+---------+-----------------------------------+------+---------------------------------+
| id   | select_type | table        | type   | possible_keys                                     | key                              | key_len | ref                               | rows | Extra                           |
+------+-------------+--------------+--------+---------------------------------------------------+----------------------------------+---------+-----------------------------------+------+---------------------------------+
|    1 | PRIMARY     | <derived2>   | ALL    | NULL                                              | NULL                             | NULL    | NULL                              | 2    | Using temporary; Using filesort |
|    1 | PRIMARY     | page         | eq_ref | PRIMARY                                           | PRIMARY                          | 4       | temp_backlink_range.pl_from       | 1    |                                 |
|    1 | PRIMARY     | translate_pp | eq_ref | PRIMARY,pp_propname_page,pp_propname_sortkey_page | PRIMARY                          | 66      | temp_backlink_range.pl_from,const | 1    | Using where; Not exists         |
|    2 | DERIVED     | pagelinks    | range  | pl_target_id,pl_backlinks_namespace_target_id     | pl_backlinks_namespace_target_id | 12      | NULL                              | 39   | Using where; Using index        |
|    2 | DERIVED     | redirect     | eq_ref | PRIMARY,rd_ns_title                               | PRIMARY                          | 4       | metawiki.pagelinks.pl_from        | 1    | Using where                     |
+------+-------------+--------------+--------+---------------------------------------------------+----------------------------------+---------+-----------------------------------+------+---------------------------------+