diff --git a/src/Handling/SpellCheckMiddleware.php b/src/Handling/SpellCheckMiddleware.php
index 00532c9..4ab5ed0 100644
--- a/src/Handling/SpellCheckMiddleware.php
+++ b/src/Handling/SpellCheckMiddleware.php
@@ -7,6 +7,7 @@
 use SilverStripe\Control\Middleware\HTTPMiddleware;
 use SilverStripe\Core\Config\Configurable;
 use SilverStripe\Forms\HTMLEditor\HTMLEditorConfig;
+use SilverStripe\Forms\HTMLEditor\TinyMCEConfig;
 use SilverStripe\i18n\i18n;
 use SilverStripe\Security\SecurityToken;
 
@@ -25,17 +26,25 @@ class SpellCheckMiddleware implements HTTPMiddleware
     public function process(HTTPRequest $request, callable $delegate)
     {
         // Set settings
-        $editor = static::config()->get('editor');
-        HTMLEditorConfig::get($editor)->enablePlugins('spellchecker');
-        HTMLEditorConfig::get($editor)->addButtonsToLine(2, 'spellchecker');
+        $editor = $this->config()->get('editor');
+
+        /** @var TinyMCEConfig $editorConfig */
+        $editorConfig = TinyMCEConfig::get($editor);
+
+        $editorConfig->enablePlugins('spellchecker');
+        $editorConfig->addButtonsToLine(2, 'spellchecker');
+
         $token = SecurityToken::inst();
-        HTMLEditorConfig::get($editor)
+
+        $editorConfig
             ->setOption('spellchecker_rpc_url', Director::absoluteURL($token->addToUrl('spellcheck/')))
             ->setOption('browser_spellcheck', false)
-            ->setOption(
-                'spellchecker_languages',
-                implode(',', $this->getLanguages())
-            );
+            ->setOption('spellchecker_languages', implode(',', $this->getLanguages()));
+
+        $defaultLocale = $this->getDefaultLocale();
+        if ($defaultLocale) {
+            $editorConfig->setOption('spellchecker_language', $defaultLocale);
+        }
 
         return $delegate($request);
     }
@@ -53,4 +62,25 @@ public function getLanguages()
         }
         return $languages;
     }
+
+    /**
+     * Returns the default locale for TinyMCE. Either via configuration or the first in the list of locales.
+     *
+     * @return string|false
+     */
+    public function getDefaultLocale()
+    {
+        // Check configuration first
+        $defaultLocale = SpellController::config()->get('default_locale');
+        if ($defaultLocale) {
+            return $defaultLocale;
+        }
+
+        // Grab the first one in the list
+        $locales = SpellController::get_locales();
+        if (empty($locales)) {
+            return false;
+        }
+        return reset($locales);
+    }
 }
diff --git a/src/Handling/SpellController.php b/src/Handling/SpellController.php
index e07deac..3169b57 100644
--- a/src/Handling/SpellController.php
+++ b/src/Handling/SpellController.php
@@ -21,7 +21,16 @@ class SpellController extends Controller
      * @var array
      * @config
      */
-    private static $locales = array();
+    private static $locales = [];
+
+    /**
+     * Optional: define the default locale for TinyMCE instances. If not defined, the first locale in the list of
+     * available locales will be used.
+     *
+     * @var string|bool
+     * @config
+     */
+    private static $default_locale = false;
 
     /**
      * Necessary permission required to spellcheck. Set to empty or null to disable restrictions.
diff --git a/tests/Handling/SpellCheckMiddlewareTest.php b/tests/Handling/SpellCheckMiddlewareTest.php
new file mode 100644
index 0000000..5dc4207
--- /dev/null
+++ b/tests/Handling/SpellCheckMiddlewareTest.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace SilverStripe\Spellcheck\Tests\Handling;
+
+use SilverStripe\Core\Config\Config;
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\SpellCheck\Handling\SpellCheckMiddleware;
+use SilverStripe\SpellCheck\Handling\SpellController;
+
+class SpellCheckMiddlewareTest extends SapphireTest
+{
+    public function testGetDefaultLocale()
+    {
+        $middleware = new SpellCheckMiddleware();
+
+        Config::modify()->set(SpellController::class, 'default_locale', 'foo');
+        $this->assertSame('foo', $middleware->getDefaultLocale(), 'Returns configured default');
+
+        Config::modify()
+            ->set(SpellController::class, 'default_locale', false)
+            ->set(SpellController::class, 'locales', ['foo_BAR', 'bar_BAZ']);
+        $this->assertSame('foo_BAR', $middleware->getDefaultLocale(), 'Returns first in `locales`');
+    }
+}