<?php

$strings = [
  'fr' => [
    'apple' => 'pomme',
    'banana' => 'banane'
  ],
  'en' => [
    'apple' => 'apple',
    'banana' => 'banana'
  ]
];

$locale = 'fr';
$getLocaleString = function($stringid) use ($strings, $locale) {
  return $strings[$locale][$stringid];
};

function _($stringid) {
  global $getLocaleString;
  echo $getLocaleString($stringid);
}

_('apple');