diff --git a/src/Forms/HTMLEditor/TinyMCEConfig.php b/src/Forms/HTMLEditor/TinyMCEConfig.php
index a2f90546c..1c06c8a09 100644
--- a/src/Forms/HTMLEditor/TinyMCEConfig.php
+++ b/src/Forms/HTMLEditor/TinyMCEConfig.php
@@ -34,7 +34,7 @@ class TinyMCEConfig extends HTMLEditorConfig
         'cy_GB' => 'cy',
         'da_DK' => 'da',
         'da_GL' => 'da',
-        'de_AT' => 'de',
+        'de_AT' => 'de_AT',
         'de_BE' => 'de',
         'de_CH' => 'de',
         'de_DE' => 'de',
@@ -56,7 +56,7 @@ class TinyMCEConfig extends HTMLEditorConfig
         'es_GQ' => 'es',
         'es_GT' => 'es',
         'es_HN' => 'es',
-        'es_MX' => 'es',
+        'es_MX' => 'es_MX',
         'es_NI' => 'es',
         'es_PA' => 'es',
         'es_PE' => 'es',
@@ -69,11 +69,9 @@ class TinyMCEConfig extends HTMLEditorConfig
         'es_AD' => 'es',
         'es_BZ' => 'es',
         'es_US' => 'es',
-        'fa_AF' => 'fa',
-        'fa_IR' => 'fa',
-        'fa_PK' => 'fa',
-        'fi_FI' => 'fi',
-        'fi_SE' => 'fi',
+        'fa_AF' => 'fa_IR',
+        'fa_IR' => 'fa_IR',
+        'fa_PK' => 'fa_IR',
         'fr_BE' => 'fr_FR',
         'fr_BF' => 'fr_FR',
         'fr_BI' => 'fr_FR',
@@ -117,12 +115,11 @@ class TinyMCEConfig extends HTMLEditorConfig
         'fr_YT' => 'fr_FR',
         'fr_GB' => 'fr_FR',
         'fr_US' => 'fr_FR',
-        'he_IL' => 'he',
-        'hu_HU' => 'hu',
-        'hu_AT' => 'hu',
-        'hu_RO' => 'hu',
-        'hu_RS' => 'hu',
-        'is_IS' => 'is',
+        'he_IL' => 'he_IL',
+        'hu_HU' => 'hu_HU',
+        'hu_AT' => 'hu_HU',
+        'hu_RO' => 'hu_HU',
+        'hu_RS' => 'hu_HU',
         'it_CH' => 'it',
         'it_IT' => 'it',
         'it_SM' => 'it',
@@ -131,28 +128,26 @@ class TinyMCEConfig extends HTMLEditorConfig
         'it_US' => 'it',
         'it_VA' => 'it',
         'ja_JP' => 'ja',
-        'ko_KP' => 'ko',
-        'ko_KR' => 'ko',
-        'ko_CN' => 'ko',
-        'mi_NZ' => 'mi_NZ',
-        'nb_NO' => 'nb',
-        'nb_SJ' => 'nb',
+        'ko_KP' => 'ko_KR',
+        'ko_KR' => 'ko_KR',
+        'ko_CN' => 'ko_KR',
+        'nb_NO' => 'nb_NO',
+        'nb_SJ' => 'nb_NO',
         'nl_AN' => 'nl',
         'nl_AW' => 'nl',
         'nl_BE' => 'nl',
         'nl_NL' => 'nl',
         'nl_SR' => 'nl',
-        'nn_NO' => 'nn',
         'pl_PL' => 'pl',
         'pl_UA' => 'pl',
-        'pt_AO' => 'pt',
-        'pt_BR' => 'pt',
-        'pt_CV' => 'pt',
-        'pt_GW' => 'pt',
-        'pt_MZ' => 'pt',
-        'pt_PT' => 'pt',
-        'pt_ST' => 'pt',
-        'pt_TL' => 'pt',
+        'pt_AO' => 'pt_PT',
+        'pt_BR' => 'pt_BR',
+        'pt_CV' => 'pt_PT',
+        'pt_GW' => 'pt_PT',
+        'pt_MZ' => 'pt_PT',
+        'pt_PT' => 'pt_PT',
+        'pt_ST' => 'pt_PT',
+        'pt_TL' => 'pt_PT',
         'ro_MD' => 'ro',
         'ro_RO' => 'ro',
         'ro_RS' => 'ro',
@@ -162,31 +157,26 @@ class TinyMCEConfig extends HTMLEditorConfig
         'ru_RU' => 'ru',
         'ru_SJ' => 'ru',
         'ru_UA' => 'ru',
-        'si_LK' => 'si',
         'sk_SK' => 'sk',
         'sk_RS' => 'sk',
-        'sq_AL' => 'sq',
-        'sr_BA' => 'sr',
-        'sr_ME' => 'sr',
-        'sr_RS' => 'sr',
-        'sv_FI' => 'sv',
-        'sv_SE' => 'sv',
+        'sv_FI' => 'sv_SE',
+        'sv_SE' => 'sv_SE',
         'tr_CY' => 'tr',
-        'tr_TR' => 'tr',
+        'tr_TR' => 'tr_TR',
         'tr_DE' => 'tr',
         'tr_MK' => 'tr',
-        'uk_UA' => 'uk',
-        'vi_VN' => 'vi',
-        'vi_US' => 'vi',
-        'zh_CN' => 'zh-cn',
-        'zh_HK' => 'zh-cn',
-        'zh_MO' => 'zh-cn',
-        'zh_SG' => 'zh-cn',
-        'zh_TW' => 'zh-tw',
-        'zh_ID' => 'zh-cn',
-        'zh_MY' => 'zh-cn',
-        'zh_TH' => 'zh-cn',
-        'zh_US' => 'zn-cn',
+        'uk_UA' => 'uk_UA',
+        'vi_VN' => 'vi_VN',
+        'vi_US' => 'vi_VN',
+        'zh_CN' => 'zh_CN',
+        'zh_HK' => 'zh_CN',
+        'zh_MO' => 'zh_CN',
+        'zh_SG' => 'zh_CN',
+        'zh_TW' => 'zh_TW',
+        'zh_ID' => 'zh_CN',
+        'zh_MY' => 'zh_CN',
+        'zh_TH' => 'zh_CN',
+        'zh_US' => 'zh_CN',
     ];
 
     /**
diff --git a/tests/php/Forms/HTMLEditor/TinyMCEConfigTest.php b/tests/php/Forms/HTMLEditor/TinyMCEConfigTest.php
new file mode 100644
index 000000000..7178945a3
--- /dev/null
+++ b/tests/php/Forms/HTMLEditor/TinyMCEConfigTest.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace SilverStripe\Forms\Tests\HTMLEditor;
+
+use SilverStripe\Control\Director;
+use SilverStripe\Core\Manifest\ModuleResourceLoader;
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\Forms\HTMLEditor\TinyMCEConfig;
+
+class TinyMCEConfigTest extends SapphireTest
+{
+    /**
+     * Ensure that all TinyMCEConfig.tinymce_lang are valid
+     */
+    public function testLanguagesValid()
+    {
+        $configDir = TinyMCEConfig::config()->get('base_dir');
+        if (!$configDir) {
+            $this->markTestSkipped("Test skipped without TinyMCE resources folder being installed");
+        }
+
+        $langs = Director::baseFolder().'/'.ModuleResourceLoader::resourcePath($configDir).'/langs';
+
+        // Test all langs exist as real files
+        foreach (TinyMCEConfig::config()->get('tinymce_lang') as $locale => $resource) {
+            // Check valid
+            $this->assertFileExists(
+                "{$langs}/{$resource}.js",
+                "Locale code {$locale} maps to {$resource}.js which exists"
+            );
+            // Check we don't simplify to locale when a specific version exists
+            if (strpos($resource, '_') === false) {
+                $this->assertFileNotExists(
+                    "{$langs}/{$locale}.js",
+                    "Locale code {$locale} doesn't map to simple {$resource}.js when a better {$locale}.js is available"
+                );
+            }
+        }
+    }
+}