diff --git a/modules/routes/routeI18n.py b/modules/routes/routeI18n.py
index 77cf7282..5b4f600b 100644
--- a/modules/routes/routeI18n.py
+++ b/modules/routes/routeI18n.py
@@ -591,6 +591,33 @@ async def create_language_set(
     return {"status": "accepted", "code": code}
 
 
+def _compute_language_sync_diff(db, code: str) -> dict:
+    """Return key sync metrics before AI translate (no DB writes)."""
+    if code == "xx":
+        raise HTTPException(status_code=400, detail="Das xx-Set wird separat synchronisiert.")
+    rows = db.getRecordset(UiLanguageSet, recordFilter={"id": code})
+    if not rows:
+        raise HTTPException(status_code=404, detail="Sprachset nicht gefunden")
+    xx_entries = _loadMasterXxEntries(db)
+    if not xx_entries:
+        raise HTTPException(status_code=503, detail="Basisset (xx) nicht vorhanden.")
+    row = dict(rows[0])
+    cur_entries = _rowEntries(row)
+    cur_by_key = {e["key"]: e for e in cur_entries}
+    xx_by_key = {e["key"]: e for e in xx_entries}
+    master_keys = set(xx_by_key.keys())
+    current_keys = set(cur_by_key.keys())
+    added_count = len(master_keys - current_keys)
+    removed_count = len(current_keys - master_keys)
+    return {
+        "code": code,
+        "addedCount": added_count,
+        "removedCount": removed_count,
+        "masterEntryCount": len(master_keys),
+        "currentEntryCount": len(current_keys),
+    }
+
+
 async def _syncLanguageWithXx(db, code: str, userId: Optional[str], adminUser: Optional[User] = None) -> dict:
     """Synchronise a language set (incl. de) against the xx base set via AI."""
     if code == "xx":
@@ -703,6 +730,19 @@ async def update_all_language_sets(
     return {"xxSync": xxSync, "updated": results}
 
 
+@router.get("/sets/{code}/sync-diff")
+async def get_language_sync_diff(
+    code: str,
+    adminUser: User = Depends(requireSysAdminRole),
+):
+    """How many keys would be added/removed vs xx before running a full sync (SysAdmin)."""
+    c = code.strip().lower()
+    if c in ("update-all", "sync-xx", "sync-de"):
+        raise HTTPException(status_code=400, detail="Ungültiger Sprachcode.")
+    db = getMgmtInterface(adminUser, mandateId=None).db
+    return _compute_language_sync_diff(db, c)
+
+
 @router.put("/sets/{code}")
 async def update_language_set(
     code: str,