Spoken Words
Given that you are writing in English, and the majority of the time your Orcs are speaking to each other in Orcish (but translated to English for the reader), then the best plan I believe is to treat both spoken Orcish and English the same, like normal dialog, and only convey, if needed, that one or the other is being spoken as relevant to the story (since it is assumed in most cases it will be Orcish translated to the reader).
On a rare occasion when you might give an actual Orcish term, there is still no need to do anything special, as the reader will pick up on it simply by not knowing what the word(s) is(are), but some hint of explanation should accompany it (or a footed/endnoted translation if it is vital):
"Gradzit! Ferbog nor taak," she greeted the newcomer in friendship.
Self-Dialog
For internal, self-dialog, the most common convention in fantasy literature is the italicizing of direct thoughts (like one speaking to oneself, or one speaking in their mind "to" someone else without that actually being known what they are saying). George R. R. Martin does this regularly in his Song of Ice and Fire series (first book is Game of Thrones), but he was by know means the first, as that convention has been around a while. This is from p.784:
Jon Snow straightened himself and took a long deep breath. Forgive me, Father. Robb. Arya. Bran ... forgive me. I cannot help you. He has the truth of it. This is my place. "I am ... yours, my lord. Your man. I swear it. I will not run again."
Telepathic
However, italics have been used for telepathic speech (when they are not used for self-dialog). An example of that is Juliet Marillier's, The Sevenwaters Triology (first book is Daughter of the Forest, p.69):
I spoke to Finbar directly, without words, mind straight to mind.
Leave this to me. Trust me.
Finbar blinked at me, relaxing his guard for a moment. I read in his thoughts an anger and confusion that I had not seen in him before.
It's not you I don't trust. It's him.
Assuming you want to use italics for self-dialog, then one solution for telepathic speech was Joel Rosenberg's use of asterisks to offset the mental speech of the dragon Ellergon in his The Guardians of the Flame series (first book The Sleeping Dragon, who is Ellergon). I don't have a copy of the series to verify it, but if some of the quote from this site is correct, then a sample is this follows (But I'm not clear from where in the book, the source site is not the best of sources):
"Ellegon!"
*Yes, I'm Ellegon. And you are Karl Cullinane.* A paw slapped against stone. *And this is a floor...*
"Enough. I take it you're the company."
*Very clever. I am also transportation. We will camp on the edge of the forest tonight. Just in case you're interested, I've spent a good part of the past year ferrying some of your possessions here, things you left at the base of Bremon. Including one red mare that emptied her bowels all the way across the Waste. I don't think she likes me. But she does look tasty.*
"We are not eating my horse. And are you certain you can carry all of us?"
*No. Actually, I just want to see how high I can get before we crash. Any other stupid questions?*
Now in that series, I believe the telepathic communication was one way (Ellergon spoke mentally, but could not read others minds, and so the characters had to verbally communicate to him, which he could understand). But the point is that you can replace the quotation marks for verbal speech with something, such as an asterisk, to carry on mental conversations. You could, if you wanted to keep all mental activity indicated by italics (which is more easily noticed), distinguish self-dialog without asterisks (or whatever marker) and telepathic communication between entities with those markers:
Hmm. I wasn't so sure that was a good idea. *Do you really mean that?*
*Yes, I really mean it* Do you think I'm an idiot, why would I not mean it.
But it may be more clear keep italics for self-dialog only, and just use the markers with normal text for telepathic:
Hmm. I wasn't so sure that was a good idea. *Do you really mean that?*
*Yes, I really mean it!* Do you think I'm an idiot, why would I not mean it.