MySQL optimizer won’t use the index

With the following query:

SELECT 
  *
FROM 
  geoname 
  LEFT JOIN alternatename
    ON geoname.geonameid = alternatename.geonameid
    AND alternatename.isPreferredName = 1 
    AND alternatename.isoLanguage = 'es'
WHERE 
  geoname.fcode = 'ADM2' 
  AND geoname.country = 'ES'

I get the following execution plan:

+----+-------------+---------------+------+-----------------------------------------+--------------------+---------+----------------------------------+------+-----------------------+
| id | select_type | table         | type | possible_keys                           | key                | key_len | ref                              | rows | Extra                 |
+----+-------------+---------------+------+-----------------------------------------+--------------------+---------+----------------------------------+------+-----------------------+
|  1 | SIMPLE      | geoname       | ref  | fcode_idx,country_idx,fcode_country_idx | fcode_country_idx  | 42      | const,const                      |   51 | Using index condition |
|  1 | SIMPLE      | alternatename | ref  | IDX_8F82EED4E2097D,idx_geoid_lang_pref  | IDX_8F82EED4E2097D | 4       | acme_geonames.geoname.geonameid  |    1 | Using where           |
+----+-------------+---------------+------+-----------------------------------------+--------------------+---------+----------------------------------+------+-----------------------+

The IDX_8F82EED4E2097D is on the geonameid field only, while the idx_geoid_lang_pref index is on the fields:

1. geonameid
2. isPreferredName
3. isoLanguage

Why doesn't MySQL use the idx_geoid_lang_pref index for the alternatename join, which I made specifically for this query?

Update as asked in comments:

the exact version of MySQL

5.6.24

what are the tables sizes?

• geoname: 4.082.489 rows
• alternatename: 2.558.890 rows

How many rows does the query return?

52

How much time does it need to run?

0:00:0.00142580 according to QueryStats

Note: my goal is not to speed up this particular request, only to understand why an index which cover the exact fields of the query is not chosen by the optimizer.

Provide the full SHOW CREATE TABLE statement for each table

CREATE TABLE `geoname` (
  `geonameid` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(200) COLLATE utf8_unicode_ci NOT NULL,
  `asciiname` varchar(200) COLLATE utf8_unicode_ci DEFAULT NULL,
  `latitude` decimal(10,7) DEFAULT NULL,
  `longitude` decimal(10,7) DEFAULT NULL,
  `fclass` char(1) COLLATE utf8_unicode_ci DEFAULT NULL,
  `fcode` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,
  `country` varchar(2) COLLATE utf8_unicode_ci DEFAULT NULL,
  `cc2` varchar(60) COLLATE utf8_unicode_ci DEFAULT NULL,
  `admin1` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `admin2` varchar(80) COLLATE utf8_unicode_ci DEFAULT NULL,
  `admin3` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `admin4` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `population` int(11) DEFAULT NULL,
  `elevation` int(11) DEFAULT NULL,
  `gtopo30` int(11) DEFAULT NULL,
  `timezone` varchar(40) COLLATE utf8_unicode_ci DEFAULT NULL,
  `moddate` date DEFAULT NULL,
  PRIMARY KEY (`geonameid`),
  KEY `fclass_idx` (`fclass`),
  KEY `fcode_idx` (`fcode`),
  KEY `country_idx` (`country`),
  KEY `admin1_idx` (`admin1`),
  KEY `admin2_idx` (`admin2`),
  KEY `fcode_country_idx` (`fcode`,`country`)
) ENGINE=InnoDB AUTO_INCREMENT=10346797 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `alternatename` (
  `alternatenameId` int(11) NOT NULL AUTO_INCREMENT,
  `geonameid` int(11) NOT NULL,
  `isoLanguage` varchar(7) COLLATE utf8_unicode_ci NOT NULL,
  `alternateName` varchar(200) COLLATE utf8_unicode_ci NOT NULL,
  `isPreferredName` tinyint(1) NOT NULL,
  `isShortName` tinyint(1) NOT NULL,
  `isColloquial` tinyint(1) NOT NULL,
  `isHistoric` tinyint(1) NOT NULL,
  PRIMARY KEY (`alternatenameId`),
  KEY `IDX_8F82EED4E2097D` (`geonameid`),
  KEY `idx_geoid_lang_pref` (`geonameid`,`isPreferredName`,`isoLanguage`),
  CONSTRAINT `FK_8F82EED4E2097D` FOREIGN KEY (`geonameid`) REFERENCES `geoname` (`geonameid`)
) ENGINE=InnoDB AUTO_INCREMENT=10842837 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;