Niklas pointed out:
MariaDB [dev_translatewiki_net]> explain SELECT translation_source_language,translation_target_language,translation_status,count(translation_target_language) as count FROM `bw_cx_translations` GROUP BY translation_source_language, translation_target_language ORDER BY translation_status, translation_source_language ;
+------+-------------+--------------------+------+---------------+------+---------+------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+--------------------+------+---------------+------+---------+------+------+---------------------------------+
| 1 | SIMPLE | bw_cx_translations | ALL | NULL | NULL | NULL | NULL | 5 | Using temporary; Using filesort |
+------+-------------+--------------------+------+---------------+------+---------+------+------+---------------------------------+
That is full table scan, dumping it into temporary file and sorting it as far as I understand. That does not scale.
We need a new index with translation_source_language, translation_target_language or change the current index to have them as prefixes.