My googling has come up empty... anyone know if one can convert common to Orc somewhere?

Would like this in Orcish:

“The Mountain That Rides” or even just “The Mountain"

Bonus points if you can guess what my build will look like

Orcs speak the same language as everyone else.

There is an orcish language. In "How Orsinium passed to the Orcs", I think it was, Gortwog is said to have trouble speaking Cyrodiilic. However, I don't believe there is an actual elaborated language by Bethesda yet.

As others mentioned, there's not really any info on their language.

The only thing I know is that the prefix on their last names is "gra" for females, and "gro" for males.

I did find this:

http://www.angelfire.com/ia/orcishnations/orcishphrases.html

Not sure where it is from but “Mountain that Rides” would be:

Mal Alag (Ajog) Bartom(s)

Doesn’t really sound a Skyrim Orc Name to me...

Reason I am trying to figure this out is I am trying to name an Orc character based off Ser Gregore Clemane of GoT... he is call “The Mountain”, and “The Mountain That Rides”...

Well none of those names have anything to with TES lore. (I'm pretty sure that page is all Tolkien)
TES Orcish is not based on any other orc language that might exist in other lore or mythology.

You can basically just invent your own name.
A good place to draw inspiration would be to look at all the Orc NPC names in Oblivion and Morrowind. The Orcs in Skyrim don't usually have last names, so you don't get the full effect of their naming method.