diff --git a/frontend/src/app/dashboard/glossaries/[id]/page.tsx b/frontend/src/app/dashboard/glossaries/[id]/page.tsx
index 322a41c..83674a2 100644
--- a/frontend/src/app/dashboard/glossaries/[id]/page.tsx
+++ b/frontend/src/app/dashboard/glossaries/[id]/page.tsx
@@ -153,10 +153,12 @@ export default function GlossaryDetailPage() {
       if (i !== index) return t;
       const translations = { ...(t.translations || {}) } as Record<string, string>;
 
-      const editLang = field === 'source' ? sourceLanguage : targetLanguage;
+      const editLang = (field === 'source' ? sourceLanguage : targetLanguage).toLowerCase();
+      const srcLang = glossary.source_language.toLowerCase();
+      const tgtLang = glossary.target_language.toLowerCase();
 
       if (field === 'source') {
-        if (editLang === glossary.source_language) {
+        if (editLang === srcLang) {
           return { ...t, source: value };
         } else {
           if (editLang && editLang !== 'multi') {
@@ -165,7 +167,7 @@ export default function GlossaryDetailPage() {
           return { ...t, translations };
         }
       } else {
-        if (editLang === 'multi' || editLang === glossary.target_language) {
+        if (editLang === 'multi' || editLang === tgtLang) {
           return { ...t, target: value };
         } else {
           if (editLang && editLang !== 'multi') {
@@ -215,12 +217,24 @@ export default function GlossaryDetailPage() {
   };
 
   const handleSourceLanguageChange = (newLang: string) => {
+    // For multilingual glossaries, the selector is a VIEW filter only.
+    // migrateTerms must not be called — it would destroy translation data.
+    if (!glossary || glossary.target_language === 'multi') {
+      setSourceLanguage(newLang);
+      return;
+    }
     const updated = migrateTerms(terms, sourceLanguage, newLang, targetLanguage, targetLanguage);
     setSourceLanguage(newLang);
     setTerms(updated);
   };
 
   const handleTargetLanguageChange = (newLang: string) => {
+    // For multilingual glossaries, the selector is a VIEW filter only.
+    // migrateTerms must not be called — it would destroy translation data.
+    if (!glossary || glossary.target_language === 'multi') {
+      setTargetLanguage(newLang);
+      return;
+    }
     const updated = migrateTerms(terms, sourceLanguage, sourceLanguage, targetLanguage, newLang);
     setTargetLanguage(newLang);
     setTerms(updated);