fix(AU-2128): Let 'en' be selected as transcript by default when youtube video (#35116)

Author: Rodra

xmodule/video_block/transcripts_utils.py

@@ -866,8 +866,11 @@ def get_default_transcript_language(self, transcripts, dest_lang=None):
         """
         sub, other_lang = transcripts["sub"], transcripts["transcripts"]
 
-        if dest_lang and dest_lang in other_lang.keys():
-            transcript_language = dest_lang
+        if dest_lang:
+            if dest_lang in other_lang.keys():
+                transcript_language = dest_lang
+            elif dest_lang == 'en' and (not other_lang or (other_lang and sub)):
+                transcript_language = 'en'
         elif self.transcript_language in other_lang:
             transcript_language = self.transcript_language
         elif sub: