diff --git a/composer.json b/composer.json
index f7c340c..cdbbba5 100644
--- a/composer.json
+++ b/composer.json
@@ -31,6 +31,7 @@
     "symfony/framework-bundle": "^7.0|^8.0",
     "symfony/yaml": "^7.0|^8.0",
     "symfony/translation": "^7.0|^8.0",
+    "ext-intl": "*",
     "assoconnect/php-quality-config": "^2.2",
     "phpstan/phpstan-symfony": "^2"
   },
diff --git a/tests/TranslationCatalogueTest.php b/tests/TranslationCatalogueTest.php
index fa4c1b8..378a4de 100644
--- a/tests/TranslationCatalogueTest.php
+++ b/tests/TranslationCatalogueTest.php
@@ -4,6 +4,8 @@
 
 namespace AssoConnect\SmtpToolbox\Tests;
 
+use IntlException;
+use MessageFormatter;
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Yaml\Yaml;
@@ -16,10 +18,10 @@ class TranslationCatalogueTest extends TestCase
 {
     private const string TRANSLATIONS_DIR = __DIR__ . '/../translations';
     private const string DOMAIN = 'assoconnect_smtp_toolbox+intl-icu';
-    private const string REFERENCE_LOCALE = 'en_US';
+    private const string REFERENCE_LOCALE = 'en';
 
     /** Every locale the catalogues are expected to cover. */
-    private const array EXPECTED_LOCALES = ['en_US', 'fr_FR', 'es_ES'];
+    private const array EXPECTED_LOCALES = ['en', 'fr', 'es'];
 
     /** @return iterable<string, array{locale: string}> */
     public static function provideLocales(): iterable
@@ -74,6 +76,44 @@ public function testEveryMessageIsTranslated(string $locale): void
         }
     }
 
+    /**
+     * The domain carries the +intl-icu suffix, so every message is an ICU pattern. `{{ name }}` is a
+     * syntax error there, and the translator throws on render rather than at load time.
+     */
+    #[DataProvider('provideLocales')]
+    public function testEveryMessageIsAValidIcuPattern(string $locale): void
+    {
+        $invalid = [];
+
+        foreach ($this->messages($locale) as $key => $message) {
+            if ('' === trim($message)) {
+                continue;
+            }
+
+            try {
+                new MessageFormatter($locale, $message);
+            } catch (IntlException $exception) {
+                $invalid[] = sprintf('%s (%s)', $key, $exception->getMessage());
+            }
+        }
+
+        self::assertSame([], $invalid, sprintf('Invalid ICU pattern(s) in the %s catalogue.', $locale));
+    }
+
+    #[DataProvider('provideLocales')]
+    public function testEveryMessageCarriesTheReferencePlaceholders(string $locale): void
+    {
+        $messages = $this->messages($locale);
+
+        foreach ($this->messages(self::REFERENCE_LOCALE) as $key => $reference) {
+            self::assertSame(
+                self::placeholders($reference),
+                self::placeholders($messages[$key] ?? ''),
+                sprintf('Message "%s" does not carry the same placeholders in %s.', $key, $locale)
+            );
+        }
+    }
+
     private static function cataloguePath(string $locale): string
     {
         return sprintf('%s/%s.%s.yml', self::TRANSLATIONS_DIR, self::DOMAIN, $locale);
@@ -113,4 +153,14 @@ private static function flatten(array $catalogue, string $prefix = ''): array
 
         return $messages;
     }
+
+    /** @return list<string> */
+    private static function placeholders(string $message): array
+    {
+        preg_match_all('/\{(\w+)\}/', $message, $matches);
+        $placeholders = array_unique($matches[1]);
+        sort($placeholders);
+
+        return $placeholders;
+    }
 }
diff --git a/translations/assoconnect_smtp_toolbox+intl-icu.en_US.yml b/translations/assoconnect_smtp_toolbox+intl-icu.en.yml
similarity index 73%
rename from translations/assoconnect_smtp_toolbox+intl-icu.en_US.yml
rename to translations/assoconnect_smtp_toolbox+intl-icu.en.yml
index f8e9778..ddf9b92 100644
--- a/translations/assoconnect_smtp_toolbox+intl-icu.en_US.yml
+++ b/translations/assoconnect_smtp_toolbox+intl-icu.en.yml
@@ -11,8 +11,8 @@ bounce:
     none: ""
     review: "Please check the email address for typo."
 invalid_address:
-  no_at_symbol: "The email address {{ email }} is missing the @ symbol"
-  no_mx_servers: "No email servers are set up for {{ domain }}"
+  no_at_symbol: "The email address {email} is missing the @ symbol"
+  no_mx_servers: "No email servers are set up for {domain}"
   unknown_user: >
-    The email address {{ email }} doesn't exist at {{ domain }} :
-    {{ smtpResponse }}
+    The email address {email} doesn't exist at {domain}:
+    {smtpResponse}
diff --git a/translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml b/translations/assoconnect_smtp_toolbox+intl-icu.es.yml
similarity index 82%
rename from translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml
rename to translations/assoconnect_smtp_toolbox+intl-icu.es.yml
index 5d5f1e6..ae2fad4 100644
--- a/translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml
+++ b/translations/assoconnect_smtp_toolbox+intl-icu.es.yml
@@ -11,8 +11,8 @@ bounce:
     none: ""
     review: "Compruebe si la dirección de correo electrónico contiene alguna errata."
 invalid_address:
-  no_at_symbol: "A la dirección {{ email }} le falta el símbolo @"
-  no_mx_servers: "No hay ningún servidor de correo configurado para {{ domain }}"
+  no_at_symbol: "A la dirección {email} le falta el símbolo @"
+  no_mx_servers: "No hay ningún servidor de correo configurado para {domain}"
   unknown_user: >
-    La dirección {{ email }} no existe en {{ domain }} :
-    {{ smtpResponse }}
+    La dirección {email} no existe en {domain}:
+    {smtpResponse}
diff --git a/translations/assoconnect_smtp_toolbox+intl-icu.fr_FR.yml b/translations/assoconnect_smtp_toolbox+intl-icu.fr.yml
similarity index 81%
rename from translations/assoconnect_smtp_toolbox+intl-icu.fr_FR.yml
rename to translations/assoconnect_smtp_toolbox+intl-icu.fr.yml
index b14b49c..ac91a42 100644
--- a/translations/assoconnect_smtp_toolbox+intl-icu.fr_FR.yml
+++ b/translations/assoconnect_smtp_toolbox+intl-icu.fr.yml
@@ -11,8 +11,8 @@ bounce:
     none: ""
     review: "Vérifiez l'adresse enregistrée : elle contient peut-être une faute de frappe."
 invalid_address:
-  no_at_symbol: "L'adresse {{ email }} ne contient pas le signe @"
-  no_mx_servers: "Aucun serveur email n'est configuré pour le domain {{ domain }}"
+  no_at_symbol: "L'adresse {email} ne contient pas le signe @"
+  no_mx_servers: "Aucun serveur email n'est configuré pour le domain {domain}"
   unknown_user: >
-    L'adresse {{ email }} n'existe pas chez {{ domain }} :
-    {{ smtpResponse }}
+    L'adresse {email} n'existe pas chez {domain} :
+    {smtpResponse}