change babel from german to ngerman

lecture.cls

@@ -4,7 +4,7 @@
 \RequirePackage[utf8]{inputenc}
 \RequirePackage[T1]{fontenc}
 \RequirePackage{textcomp}
-\RequirePackage[german]{babel}
+\RequirePackage[ngerman]{babel}
 \RequirePackage{amsmath, amssymb, amsthm}
 \RequirePackage{mdframed}
 \RequirePackage{geometry}