Translate your website

<nav aria-label="Language">
  <a href="/?lang=en" hreflang="en" lang="en">English</a>
  <a href="/?lang=fr" hreflang="fr" lang="fr">Français</a>
  <a href="/?lang=yo" hreflang="yo" lang="yo">Yorùbá</a>
</nav>

<?php return 'nt_live_…';

// nt-key.php is one folder above this website's folder
$key = require __DIR__ . '/../nt-key.php';

// In neo_translate.php, instead of getenv('NEO_TRANSLATE_KEY'):
'Authorization: Bearer ' . $key,

<!-- On every language version of the page, list them all, including itself -->
<link rel="alternate" hreflang="en" href="https://example.org/en/about/">
<link rel="alternate" hreflang="fr" href="https://example.org/fr/about/">
<link rel="alternate" hreflang="yo" href="https://example.org/yo/about/">
<link rel="alternate" hreflang="x-default" href="https://example.org/about/">

<!-- And say which language this page is in -->
<html lang="fr">

<?php
// neo_translate.php: translate with a cache. PHP 7.4 or later, with curl.
// Your key comes from the server's settings (NEO_TRANSLATE_KEY), never from a web page.

const NEO_API = 'https://api.neotranslate.org/v1/translate';
const SOURCE_LANG = 'en'; // the language your site is written in
define('CACHE_DIR', getenv('TRANSLATION_CACHE_DIR') ?: __DIR__ . '/translations');

// One file per piece of text and language. The text is part of the name, so
// changed text is translated again, and unchanged text never is.
function neo_cache_file(string $text, string $to): string {
    return CACHE_DIR . '/' . $to . '/' . hash('sha256', SOURCE_LANG . '|' . $to . '|' . $text) . '.json';
}

function neo_save(string $text, string $to, string $translation): void {
    $file = neo_cache_file($text, $to);
    if (!is_dir(dirname($file))) mkdir(dirname($file), 0775, true);
    file_put_contents($file, json_encode(['text' => $text, 'translation' => $translation]));
}

function neo_call(array $body): array {
    for ($attempt = 1; ; $attempt++) {
        $wait = 5;
        $ch = curl_init(NEO_API);
        curl_setopt_array($ch, [
            CURLOPT_POST => true,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_TIMEOUT => 30,
            CURLOPT_HTTPHEADER => [
                'Authorization: Bearer ' . getenv('NEO_TRANSLATE_KEY'),
                'Content-Type: application/json',
            ],
            CURLOPT_POSTFIELDS => json_encode(['from' => SOURCE_LANG] + $body),
            CURLOPT_HEADERFUNCTION => function ($ch, $line) use (&$wait) {
                if (stripos($line, 'Retry-After:') === 0) $wait = (int) trim(substr($line, 12));
                return strlen($line);
            },
        ]);
        $raw = curl_exec($ch);
        $status = curl_getinfo($ch, CURLINFO_RESPONSE_CODE);
        curl_close($ch);
        $data = json_decode((string) $raw, true);
        if ($status === 200) return $data;
        if ($status === 429 && $attempt < 3) { sleep(max(1, $wait)); continue; } // slow down, then try again
        throw new RuntimeException($data['error']['message'] ?? 'Translation failed (' . $status . ')');
    }
}

/** One piece of text in one language: from the cache, or translated once and kept. */
function neo_translate(string $text, string $to): string {
    if ($to === SOURCE_LANG || trim($text) === '') return $text;
    $file = neo_cache_file($text, $to);
    if (is_file($file)) return json_decode(file_get_contents($file), true)['translation'];
    $data = neo_call(['text' => $text, 'to' => $to]);
    neo_save($text, $to, $data['translation']);
    return $data['translation'];
}

/** When you publish: one piece of text into your main languages. Only missing ones are asked for. */
function neo_pretranslate(string $text, array $languages): int {
    $missing = array_values(array_filter($languages, function ($to) use ($text) {
        return $to !== SOURCE_LANG && !is_file(neo_cache_file($text, $to));
    }));
    $saved = 0;
    foreach (array_chunk($missing, 10) as $group) { // up to 10 languages a request
        $data = neo_call(['text' => $text, 'to' => $group]);
        foreach ($data['translations'] as $t) {
            if (isset($t['error'])) continue; // not available right now: the next run tries again
            neo_save($text, $t['to'], $t['translation']);
            $saved++;
        }
    }
    return $saved;
}

// In a page:        echo htmlspecialchars(neo_translate('Welcome to our church', $lang));
// When you publish: neo_pretranslate($paragraph, ['fr', 'yo', 'sw', 'pt']);

<?php
// choose_language.php: pick the visitor's language, and remember their choice.

const SUPPORTED = ['en', 'fr', 'yo', 'sw', 'pt']; // the languages your site is published in
const DEFAULT_LANG = 'en';
const LANG_COOKIE = 'lang';
// The country header your hosting or network provider adds. Ask them for its name.
const COUNTRY_HEADER = 'HTTP_X_COUNTRY';
// A hint only: most countries have many languages, and people travel.
const BY_COUNTRY = ['FR' => 'fr', 'CI' => 'fr', 'CM' => 'fr', 'BJ' => 'fr', 'KE' => 'sw',
                    'TZ' => 'sw', 'BR' => 'pt', 'PT' => 'pt', 'AO' => 'pt', 'MZ' => 'pt'];

/** "yo" or "YO" -> "yo", if your site has it; otherwise null. */
function supported_lang(?string $code): ?string {
    foreach (SUPPORTED as $s) {
        if ($code !== null && strcasecmp($s, $code) === 0) return $s;
    }
    return null;
}

/** "fr-CH, fr;q=0.9, en;q=0.8" -> ['fr-ch', 'fr', 'en'], most wanted first. */
function browser_languages(string $header): array {
    $langs = [];
    foreach (explode(',', $header) as $i => $part) {
        $bits = array_map('trim', explode(';', $part));
        $q = 1.0;
        foreach (array_slice($bits, 1) as $p) {
            if (strpos($p, 'q=') === 0) $q = (float) substr($p, 2);
        }
        if ($bits[0] !== '' && $bits[0] !== '*' && $q > 0) $langs[] = [strtolower($bits[0]), $q, $i];
    }
    usort($langs, function ($a, $b) { return [$b[1], $a[2]] <=> [$a[1], $b[2]]; });
    return array_column($langs, 0);
}

function choose_language(?string $cookie, string $acceptLanguage, string $country): string {
    if ($lang = supported_lang($cookie)) return $lang;                     // 1. their own choice
    foreach (browser_languages($acceptLanguage) as $tag) {                  // 2. their browser
        if ($lang = supported_lang($tag)) return $lang;
        if ($lang = supported_lang(explode('-', $tag)[0])) return $lang;
    }
    return supported_lang(BY_COUNTRY[strtoupper($country)] ?? null) ?? DEFAULT_LANG; // 3. where they are
}

/** The language switcher sends ?lang=fr: remember it for a year. */
function remember_language(string $lang): void {
    if (supported_lang($lang)) {
        setcookie(LANG_COOKIE, $lang, ['expires' => time() + 365 * 86400, 'path' => '/', 'samesite' => 'Lax']);
    }
}

// On your home page (an address without a language), send the visitor to theirs:
//   if (isset($_GET['lang'])) remember_language($_GET['lang']);
//   $lang = choose_language($_GET['lang'] ?? $_COOKIE[LANG_COOKIE] ?? null,
//                           $_SERVER['HTTP_ACCEPT_LANGUAGE'] ?? '', $_SERVER[COUNTRY_HEADER] ?? '');
//   header('Location: /' . $lang . '/', true, 302);