From Lord of the Rings Appendix F:
And these creatures, being filled with malice, hating even their own kind, quickly developed as many barbarous dialects as there were groups or settlements of their race, so that their Orkish speech was of little use to them in intercourse between different tribes.
So it was that in the Third Age Orcs used for communication between breed and breed the Westron tongue...
There is therefore no single Orkish language. The situation in the LotR movies is consistent with this, because - if you check the movie scenes again - you'll see that for the most part the Orcs depicted are from different tribes; for example (from The Uruk-Hai):
To Pippin's surprise he found that much of the talk was intelligible; many of the Orcs were using ordinary language. Apparently the members of two or three quite different tribes were present, and they could not understand one another's orc-speech.
It's therefore correct that they use the common speech when dealing with each other.
In the Hobbit movies however the majority of Orcs depicted are from Dol Guldur, and so it's also consistent that they would all use the same language.
