问题是您创建了一个非关联数组，但将其视为关联数组。

while($row = $result->fetch_assoc()) {
    $countryResult[] = $row["name"];
    $countryResult[] = $row["departement_code"];
}

在这个循环之后$countryResult数组会像这样

array(6) {
  [0] => "name1"
  [1] => "departement_code1"
  [2] => "name2"
  [3] => "departement_code2"
  [4] => "name3"
  [5] => "departement_code3"
}

您应该更改while循环：

while($row = $result->fetch_assoc()) {
    $countryResult[] = $row;
}