diff --git a/tests/TranslationCatalogueTest.php b/tests/TranslationCatalogueTest.php
new file mode 100644
index 0000000..fa4c1b8
--- /dev/null
+++ b/tests/TranslationCatalogueTest.php
@@ -0,0 +1,116 @@
+<?php
+
+declare(strict_types=1);
+
+namespace AssoConnect\SmtpToolbox\Tests;
+
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Yaml\Yaml;
+
+/**
+ * Every catalogue must cover the reference locale's messages, each in a wording of its own. Neither a
+ * missing catalogue nor an untranslated message fails at runtime, so this test is what guards them.
+ */
+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';
+
+    /** Every locale the catalogues are expected to cover. */
+    private const array EXPECTED_LOCALES = ['en_US', 'fr_FR', 'es_ES'];
+
+    /** @return iterable<string, array{locale: string}> */
+    public static function provideLocales(): iterable
+    {
+        foreach (self::EXPECTED_LOCALES as $locale) {
+            yield $locale => ['locale' => $locale];
+        }
+    }
+
+    #[DataProvider('provideLocales')]
+    public function testCatalogueExists(string $locale): void
+    {
+        self::assertFileExists(self::cataloguePath($locale));
+    }
+
+    #[DataProvider('provideLocales')]
+    public function testCatalogueCoversEveryReferenceMessage(string $locale): void
+    {
+        self::assertSame(
+            array_keys($this->messages(self::REFERENCE_LOCALE)),
+            array_keys($this->messages($locale)),
+            sprintf('The %s catalogue does not cover the same messages as the %s one.', $locale, self::REFERENCE_LOCALE)
+        );
+    }
+
+    #[DataProvider('provideLocales')]
+    public function testEveryMessageIsTranslated(string $locale): void
+    {
+        $messages = $this->messages($locale);
+
+        foreach ($this->messages(self::REFERENCE_LOCALE) as $key => $reference) {
+            $message = trim($messages[$key] ?? '');
+
+            // Some messages are deliberately blank, so emptiness is only a gap where the reference has wording
+            if ('' === trim($reference)) {
+                self::assertSame('', $message, sprintf('Message "%s" should stay blank.', $key));
+                continue;
+            }
+
+            self::assertNotSame('', $message, sprintf('Message "%s" has no %s translation.', $key, $locale));
+
+            if (self::REFERENCE_LOCALE === $locale) {
+                continue;
+            }
+
+            // A catalogue copied from the reference locale and left untranslated clears the emptiness check
+            self::assertNotSame(
+                trim($reference),
+                $message,
+                sprintf('Message "%s" still reads as its %s wording.', $key, self::REFERENCE_LOCALE)
+            );
+        }
+    }
+
+    private static function cataloguePath(string $locale): string
+    {
+        return sprintf('%s/%s.%s.yml', self::TRANSLATIONS_DIR, self::DOMAIN, $locale);
+    }
+
+    /** @return array<string, string> */
+    private function messages(string $locale): array
+    {
+        $catalogue = Yaml::parseFile(self::cataloguePath($locale));
+        self::assertIsArray($catalogue, sprintf('The %s catalogue is not a YAML mapping.', $locale));
+
+        $messages = self::flatten($catalogue);
+        ksort($messages);
+
+        return $messages;
+    }
+
+    /**
+     * @param array<mixed> $catalogue
+     *
+     * @return array<string, string>
+     */
+    private static function flatten(array $catalogue, string $prefix = ''): array
+    {
+        $messages = [];
+
+        foreach ($catalogue as $key => $value) {
+            $path = '' === $prefix ? (string) $key : $prefix . '.' . $key;
+
+            if (is_array($value)) {
+                $messages += self::flatten($value, $path);
+                continue;
+            }
+
+            $messages[$path] = (string) $value;
+        }
+
+        return $messages;
+    }
+}
diff --git a/translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml b/translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml
new file mode 100644
index 0000000..5d5f1e6
--- /dev/null
+++ b/translations/assoconnect_smtp_toolbox+intl-icu.es_ES.yml
@@ -0,0 +1,18 @@
+bounce:
+  reason:
+    inactive: "El buzón del destinatario ha sido desactivado."
+    quota: "El buzón del destinatario está lleno y no puede aceptar nuevos correos electrónicos por el momento"
+    none: "No hemos podido interpretar el motivo del fallo de envío comunicado por el servidor de correo"
+    other: ""
+    spam: "El sistema de correo del destinatario ha marcado el mensaje como spam y se niega a entregarlo."
+    unknown_user: "Esta dirección de correo electrónico no existe."
+  tips:
+    contact: "Informe al destinatario de este problema para que pueda resolverse."
+    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 }}"
+  unknown_user: >
+    La dirección {{ email }} no existe en {{ domain }} :
+    {{ smtpResponse }}