MDL-26804 core_string_manager::get_list_of_translations() can use a cache again
This patch reimplements the internal cache that was used to store the list of available translations in Moodle 1.x. By default, the method get_list_of_translations() still uses the file moodledata/cache/languages to store the list of available translations. The location of that file can be redefined in config.php. The internal format of the cache file is JSON now (used to be a plain text list). The patch also fixes a usage of the global $CFG in translation_exists() methods where the internal property should be used instead.