Hello!
I found a bug (and temporary solution).
I’m Polish and my copy of Halo: TMCC is of course in polish.
When i tried to play Halo 3 ODST DLC it always show me that my dlc is not installed.
After many attempts (uninstalling, reinstalling, removing to external drive, console hard reset, installing from disc, installing digital version from a game pass) I almost give up.
I didn’t find any solution because nobody here (en-us market) have the same problem.
So i understand that problem exist only on language side.
I’ve changed language from polish to english in audio menu.
Game automatically starded downloading a language pack and now everything works…almost.
Almost - because if You want play other parts in your native (non-english) language and then again you want to play ODST you have to download a language pack again (about 800MB).
I believe that this problem could be fixed by a quick patch.
P.S. Sorry for my english 