locale

Basic settings

return [
  'locale' => 'en_GB.utf-8'
];

Detailed settings

return [
  'locale' => [
    LC_ALL      => 'tr_TR.utf-8',
    LC_COLLATE  => 'tr_TR.utf-8',
    LC_MONETARY => 'tr_TR.utf-8',
    LC_NUMERIC  => 'tr_TR.utf-8',
    LC_TIME     => 'tr_TR.utf-8',
    LC_MESSAGES => 'tr_TR.utf-8',
    LC_CTYPE    => 'en_US.utf-8'
  ]
];

You can also define the locale names as strings (for example if you are loading the confguration from a JSON file):

return [
  'locale' => [
    'LC_ALL'      => 'tr_TR.utf-8',
    'LC_COLLATE'  => 'tr_TR.utf-8',
    'LC_MONETARY' => 'tr_TR.utf-8',
    'LC_NUMERIC'  => 'tr_TR.utf-8',
    'LC_TIME'     => 'tr_TR.utf-8',
    'LC_MESSAGES' => 'tr_TR.utf-8',
    'LC_CTYPE'    => 'en_US.utf-8'
  ]
];

The LC_ALL locale should always be set as it is used as a default locale internally in the Kirby core. Other locales can be set to override the default LC_ALL locale for specific use-cases.