Use unmodified English for period conversion

composer.json

@@ -40,7 +40,10 @@
     },
     "config": {
         "process-timeout": 0,
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "phpstan/extension-installer": true
+        }
     },
     "extra": {
         "branch-alias": {

phpmd.xml

@@ -33,11 +33,6 @@
         <exclude name="CouplingBetweenObjects" />
         <exclude name="CountInLoopExpression" />
     </rule>
-    <rule ref="rulesets/design.xml/CouplingBetweenObjects">
-        <properties>
-            <property name="maximum" value="25" />
-        </properties>
-    </rule>
     <rule ref="rulesets/naming.xml/ShortVariable">
         <properties>
             <property name="exceptions" value="ci,id,to,tz" />

src/Carbon/AbstractTranslator.php

@@ -68,12 +68,13 @@ abstract class AbstractTranslator extends Translation\Translator
     public static function get($locale = null)
     {
         $locale = $locale ?: 'en';
+        $key = static::class === Translator::class ? $locale : static::class.'|'.$locale;
 
-        if (!isset(static::$singletons[$locale])) {
-            static::$singletons[$locale] = new static($locale);
+        if (!isset(static::$singletons[$key])) {
+            static::$singletons[$key] = new static($locale);
         }
 
-        return static::$singletons[$locale];
+        return static::$singletons[$key];
     }
 
     public function __construct($locale, MessageFormatterInterface $formatter = null, $cacheDir = null, $debug = false)

src/Carbon/CarbonPeriod.php

@@ -163,6 +163,8 @@
  * @method $this floorMicroseconds(float $precision = 1) Truncate the current instance microsecond with given precision.
  * @method $this ceilMicrosecond(float $precision = 1) Ceil the current instance microsecond with given precision.
  * @method $this ceilMicroseconds(float $precision = 1) Ceil the current instance microsecond with given precision.
+ *
+ * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
 class CarbonPeriod implements Iterator, Countable, JsonSerializable
 {
@@ -2179,7 +2181,10 @@ public function ceilUnit($unit, $precision = 1)
      */
     public function round($precision = null, $function = 'round')
     {
-        return $this->roundWith($precision ?? (string) $this->getDateInterval(), $function);
+        return $this->roundWith(
+            $precision ?? $this->getDateInterval()->setLocalTranslator(TranslatorImmutable::get('en'))->forHumans(),
+            $function
+        );
     }
 
     /**

src/Carbon/TranslatorImmutable.php

@@ -0,0 +1,99 @@
+<?php
+
+/**
+ * This file is part of the Carbon package.
+ *
+ * (c) Brian Nesbitt <brian@nesbot.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Carbon;
+
+use Carbon\Exceptions\ImmutableException;
+use Symfony\Component\Config\ConfigCacheFactoryInterface;
+use Symfony\Component\Translation\Formatter\MessageFormatterInterface;
+
+class TranslatorImmutable extends Translator
+{
+    /** @var bool */
+    private $constructed = false;
+
+    public function __construct($locale, MessageFormatterInterface $formatter = null, $cacheDir = null, $debug = false)
+    {
+        parent::__construct($locale, $formatter, $cacheDir, $debug);
+        $this->constructed = true;
+    }
+
+    /**
+     * @codeCoverageIgnore
+     */
+    public function setDirectories(array $directories)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        return parent::setDirectories($directories);
+    }
+
+    public function setLocale($locale)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        return parent::setLocale($locale);
+    }
+
+    /**
+     * @codeCoverageIgnore
+     */
+    public function setMessages($locale, $messages)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        return parent::setMessages($locale, $messages);
+    }
+
+    /**
+     * @codeCoverageIgnore
+     */
+    public function setTranslations($messages)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        return parent::setTranslations($messages);
+    }
+
+    /**
+     * @codeCoverageIgnore
+     */
+    public function setConfigCacheFactory(ConfigCacheFactoryInterface $configCacheFactory)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        parent::setConfigCacheFactory($configCacheFactory);
+    }
+
+    public function resetMessages($locale = null)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        return parent::resetMessages($locale);
+    }
+
+    /**
+     * @codeCoverageIgnore
+     */
+    public function setFallbackLocales(array $locales)
+    {
+        $this->disallowMutation(__METHOD__);
+
+        parent::setFallbackLocales($locales);
+    }
+
+    private function disallowMutation($method)
+    {
+        if ($this->constructed) {
+            throw new ImmutableException($method.' not allowed on '.static::class);
+        }
+    }
+}

tests/CarbonPeriod/ToDatePeriodTest.php

@@ -13,7 +13,9 @@
 
 namespace Tests\CarbonPeriod;
 
+use Carbon\CarbonInterval;
 use Carbon\CarbonPeriod;
+use Carbon\Translator;
 use DatePeriod;
 use Tests\AbstractTestCase;
 
@@ -58,4 +60,41 @@ public function testToArrayIsNotEmptyArray()
         $this->assertSame('2021-01-05', $newInstance->getStartDate()->format('Y-m-d'));
         $this->assertSame(3, $newInstance->getRecurrences());
     }
+
+    public function testWithIntervalLocalized()
+    {
+        CarbonInterval::setLocale('fr');
+        $period = CarbonPeriod::create('2021-01-05', 3);
+        $result = $period->floor()->toDatePeriod();
+
+        $this->assertSame(DatePeriod::class, \get_class($result));
+        $this->assertSame('2021-01-05', $result->getStartDate()->format('Y-m-d'));
+        $this->assertNull($result->getEndDate());
+
+        if (method_exists($result, 'getRecurrences')) {
+            $this->assertSame(3, $result->getRecurrences());
+        }
+
+        CarbonInterval::setLocale('en');
+    }
+
+    public function testWithModifiedEnglish()
+    {
+        $translator = Translator::get('en');
+        $translator->setTranslations([
+            'day' => ':count boring day|:count boring days',
+        ]);
+        $period = CarbonPeriod::create('2021-01-05', 3);
+        $result = $period->floor()->toDatePeriod();
+
+        $this->assertSame(DatePeriod::class, \get_class($result));
+        $this->assertSame('2021-01-05', $result->getStartDate()->format('Y-m-d'));
+        $this->assertNull($result->getEndDate());
+
+        if (method_exists($result, 'getRecurrences')) {
+            $this->assertSame(3, $result->getRecurrences());
+        }
+
+        $translator->resetMessages();
+    }
 }

tests/Language/TranslatorTest.php

@@ -14,8 +14,10 @@
 namespace Tests\Language;
 
 use Carbon\Carbon;
+use Carbon\Exceptions\ImmutableException;
 use Carbon\Exceptions\NotLocaleAwareException;
 use Carbon\Translator;
+use Carbon\TranslatorImmutable;
 use ReflectionMethod;
 use Tests\AbstractTestCase;
 
@@ -76,4 +78,12 @@ public function testNotLocaleAwareException()
             $exception->getMessage()
         );
     }
+
+    public function testTranslatorImmutable()
+    {
+        $this->expectExceptionObject(
+            new ImmutableException('setTranslations not allowed on '.TranslatorImmutable::class)
+        );
+        TranslatorImmutable::get('en')->setTranslations([]);
+    }
 }