french Installation of civicrm 5.27.2 on drupal 8
If you've already installed civi and you're able to get to the administer - localization - languages page, then the following is probably the simplest at this point:
In civicrm.settings.php (in web/sites/default), add this line to point to the l10n folder:
define('CIVICRM_L10N_BASEDIR', '/full/path/to/l10n');
OR
Move your l10n folder into vendor/civicrm/civicrm-core.
The first is slightly better because you can put l10n somewhere that won't get deleted when you upgrade, so then I'd also move l10n somewhere completely outside civi.