Why settings.default.json file is neglected during install?
The solution was:


Add a define('CIVICRM_L10N_BASEDIR', '/var/aegir/platforms/YOUR-PATH'); statement to platform.settings.php file of Aegir (see: Altering Aegir's Behaviours), to set an environmental variable for CiviCRM, to locate the exact place of its l10n files. More info on CIVICRM_L10N_BASEDIR here and here.
Correct the line breaks of the example settings.default.json, otherwise it's not being read as JSON file. The address_format and mailing_format should be one line, and \n have to be added when line break is required. See MR.